Thank you kindly, TLaukkan (Tommil) for a patch that:

Created nunit test for LSL API and example test for 
llAngleBetween which was marked untested in wiki. 
Run new test succesfully with NUnitGUI and nant build.

OpenSim/Region/ScriptEngine/Shared/Tests/LSL_ApiTest.cs

@@ -0,0 +1,140 @@
+/*
+ * Copyright (c) Contributors, http://opensimulator.org/
+ * See CONTRIBUTORS.TXT for a full list of copyright holders.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name of the OpenSim Project nor the
+ *       names of its contributors may be used to endorse or promote products
+ *       derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE CONTRIBUTORS BE LIABLE FOR ANY
+ * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+using System.Collections.Generic;
+using NUnit.Framework;
+using OpenSim.Tests.Common;
+using OpenSim.Region.ScriptEngine.Shared;
+using OpenSim.Tests.Common.Setup;
+using OpenSim.Region.Framework.Scenes;
+using Nini.Config;
+using OpenSim.Region.ScriptEngine.Shared.Api;
+using OpenMetaverse;
+using System;
+
+namespace OpenSim.Region.ScriptEngine.Shared.Tests
+{
+    /// <summary>
+    /// Tests for LSL_Api
+    /// </summary>
+    [TestFixture]
+    public class LSL_ApiTest
+    {
+
+        private const double ANGLE_ACCURACY_IN_RADIANS = 1E-7;
+        private LSL_Api lslApi;
+
+        [SetUp]
+        public void SetUp()
+        {
+
+            IniConfigSource initConfigSource = new IniConfigSource();
+            IConfig config = initConfigSource.AddConfig("XEngine");
+            config.Set("Enabled", "true");
+
+            Scene scene = SceneSetupHelpers.SetupScene();
+            SceneObjectPart part = SceneSetupHelpers.AddSceneObject(scene);
+
+            XEngine.XEngine engine = new XEngine.XEngine();
+            engine.Initialise(scene, initConfigSource);
+
+            lslApi = new LSL_Api();
+            lslApi.Initialize(engine, part, part.LocalId, part.UUID);
+
+        }
+
+        [Test]
+        public void TestllAngleBetween()
+        {
+            TestllAngleBetween(new Vector3(1, 0, 0), 0);
+            TestllAngleBetween(new Vector3(1, 0, 0), 90);
+            TestllAngleBetween(new Vector3(1, 0, 0), 180);
+            TestllAngleBetween(new Vector3(1, 0, 0), 270);
+
+            TestllAngleBetween(new Vector3(0, 1, 0), 0);
+            TestllAngleBetween(new Vector3(0, 1, 0), 90);
+            TestllAngleBetween(new Vector3(0, 1, 0), 180);
+            TestllAngleBetween(new Vector3(0, 1, 0), 270);
+
+            TestllAngleBetween(new Vector3(0, 0, 1), 0);
+            TestllAngleBetween(new Vector3(0, 0, 1), 90);
+            TestllAngleBetween(new Vector3(0, 0, 1), 180);
+            TestllAngleBetween(new Vector3(0, 0, 1), 270);
+
+            TestllAngleBetween(new Vector3(1, 1, 1), 0);
+            TestllAngleBetween(new Vector3(1, 1, 1), 90);
+            TestllAngleBetween(new Vector3(1, 1, 1), 180);
+            TestllAngleBetween(new Vector3(1, 1, 1), 270);
+        }
+
+        private void TestllAngleBetween(Vector3 axis,float originalAngle)
+        {
+            Quaternion rotation1 = Quaternion.CreateFromAxisAngle(axis, 0);
+            Quaternion rotation2 = Quaternion.CreateFromAxisAngle(axis, ToRadians(originalAngle));
+
+            double deducedAngle = FromLslFloat(lslApi.llAngleBetween(ToLslQuaternion(rotation2), ToLslQuaternion(rotation1)));
+
+            Assert.Greater(deducedAngle, ToRadians(originalAngle) - ANGLE_ACCURACY_IN_RADIANS);
+            Assert.Less(deducedAngle, ToRadians(originalAngle) + ANGLE_ACCURACY_IN_RADIANS);
+        }
+
+        #region Conversions to and from LSL_Types
+
+        private float ToRadians(double degrees)
+        {
+            return (float)(Math.PI * degrees / 180);
+        }
+
+        private double FromRadians(float radians)
+        {
+            return radians * 180 / Math.PI;
+        }
+
+        private double FromLslFloat(LSL_Types.LSLFloat lslFloat)
+        {
+            return lslFloat.value;
+        }
+
+        private LSL_Types.LSLFloat ToLslFloat(double value)
+        {
+            return new LSL_Types.LSLFloat(value);
+        }
+
+        private Quaternion FromLslQuaternion(LSL_Types.Quaternion lslQuaternion)
+        {
+            return new Quaternion((float)lslQuaternion.x, (float)lslQuaternion.y, (float)lslQuaternion.z, (float)lslQuaternion.s);
+        }
+
+        private LSL_Types.Quaternion ToLslQuaternion(Quaternion quaternion)
+        {
+            return new LSL_Types.Quaternion(quaternion.X, quaternion.Y, quaternion.Z, quaternion.W);
+        }
+
+        #endregion
+
+    }
+}

prebuild.xml

@@ -2721,9 +2721,15 @@
 
       <ReferencePath>../../../../../bin/</ReferencePath>
       <Reference name="System" localCopy="false"/>
+      <Reference name="OpenSim.Framework"/>
+      <Reference name="OpenSim.Region.Framework"/>
       <Reference name="OpenSim.Region.ScriptEngine.Shared"/>
+      <Reference name="OpenSim.Region.ScriptEngine.Shared.Api"/>
+      <Reference name="OpenSim.Region.ScriptEngine.XEngine"/>
       <Reference name="OpenSim.Tests.Common"/>
       <Reference name="nunit.framework.dll" />
+      <Reference name="Nini.dll" />
+      <Reference name="OpenMetaverseTypes.dll"/>
       <Files>
         <Match pattern="*.cs" recurse="false"/>
       </Files>
@@ -2818,3 +2824,4 @@
   </Solution>
 </Prebuild>
 
+