Adding design-time support

1). They're not the really the same. 

Tooling wants to uniquely and unambiguously identify individual tests. We can accomplish this by giving a uniquified fully-qualified name to VS, and then letting VS give it back to us verbatim.

If you look at: https://github.com/xunit/xunit/blob/master/src/xunit.runner.utility/Project/XunitFilters.cs - `-testname` is coupled to the test 'display name', which isn't really suited for our use as a unique identifier.

2). Tooling already uses these specific commandline arguments. Changing them is a VS change.