Ported some of the python fuzz tests to C#

* Refactored the test runner to use attribute based test discovery
* Ported value and vtable/object fuzzing tests from python to C#

tests/FlatBuffers.Test/FlatBuffersTestClassAttribute.cs

@@ -0,0 +1,12 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace FlatBuffers.Test
+{
+    [AttributeUsage(AttributeTargets.Class)]
+    public class FlatBuffersTestClassAttribute : Attribute
+    {
+    }
+}