C#: Add MSTEST test project and check that the call to vstest doesn't get the UseSharedCompilation=false flag forwarded.

csharp/ql/integration-tests/posix-only/dotnet_test_mstest/UnitTest1.cs

@@ -0,0 +1,10 @@
+namespace dotnet_test_mstest;
+
+[TestClass]
+public class UnitTest1
+{
+    [TestMethod]
+    public void TestMethod1()
+    {
+    }
+}