[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/README.md

@@ -0,0 +1,38 @@
+# .NET Tests
+
+## Running on Linux
+
+### Prerequisites
+To run the tests on a Linux a few prerequisites are needed:
+
+1) mono
+2) msbuild
+
+### Running
+
+To run the tests:
+
+```
+./NetTest.sh
+```
+
+This will download the .NET installer and core SDK if those are not already
+installed. Then it will build the tests using `msbuild` and run the resulting
+test binary with `mono`.
+
+After running the tests, the downloaded .NET installer and SDK are *not* removed
+as they can be reused in subsequent invocations. The files are ignored by git by
+default, and can remain in the working directory.
+
+### Cleaning
+
+If you want to clean up the downloaded .NET installer and SDK, run:
+
+```
+./clean.sh
+```
+
+This will wipe away the downloaded files and directories. Those will be
+automatically re-downloaded when running `NetTest.sh`.
+
+