Convert all tests to use xunit

Changes the test framework from mstest to xunit without changing the behavior of existing tests.
Merge pull request #63 from heaths/xunit
Update package restore to acquire xunit v1.9.2
Add AllBuilt.testproj to test with all test assemblies.

The test.proj is a traversal project and should not have additional
targets. Create AllBuilt.testproj and move the test.proj Test target
to execute all test assemblies built by the WiX toolset.