[C#] Cleaned up .NET testing script for Mono (#6016)

* Cleaned up .NET testing script for Mono

Cleaned up the .NET testing script to make it a little better. It
purposefully doesn't delete the .NET installer and SDk after running the
script, so that they can be used in subsequent invocations. This greatly
speeds up the script.

The downloaded files are ignored by git by default. They can be
explicitly cleaned up by runnning the clean script (clean.sh).

* Trying using older Version indicator

* Remove lins to monsterdata and reference it directly.

* Updated appveryor script to remove copying of files no longer needed

* Continue to update appveyor script to work. Disabled CS0169 in ByteBufferTest

tests/FlatBuffers.Test/ByteBufferTests.cs

@@ -591,11 +591,14 @@ namespace FlatBuffers.Test
             Assert.Throws<ArgumentException>(() => uut.Put(1024, data));
         }
 
+        #pragma warning disable 0169
+        // These are purposely not used and the warning is suppress
         private struct dummyStruct
         {
             int a;
             float b;
         }
+        #pragma warning restore 0169
 
         [FlatBuffersTestMethod]
         public void ByteBuffer_Put_Array_IncorrectType_Throws()