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/FlatBuffersTestMethodAttribute.cs

@@ -0,0 +1,9 @@
+using System;
+
+namespace FlatBuffers.Test
+{
+    [AttributeUsage(AttributeTargets.Method)]
+    public class FlatBuffersTestMethodAttribute : Attribute
+    {
+    }
+}