Ported xunit and xunit.extensions tests to test.xunit1 to verify v2 runners running v1 tests.
Move all xUnit v1 tests into namespaces so they can be easily segregated.
Moved xUnit v2 runner classes into v2 folder.
MSBuild runner cleanup.
Rename parameter name in ITestFrameworkExecutor.
Make message implementations internal so they can be shared between xunit2.dll and xunit.runner.utility.dll.
Enable xUnit v1 tests to be run by v2 runners.