switch mstest projects to use xunit

[kayle]

Fixes #207

[msftclas]

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[msftclas]

@kayle, thanks for signing the contribution license agreement. We will now validate the agreement and then the pull request.

Thanks, Microsoft Pull Request Bot

[sharwell]

This change will definitely make #209 easier. It also moves the test suite in a direction of consistency with other open source Microsoft projects. The change to ETW_GeneralParsing_Basic could be seen as a concern, but I do not view it as such since our AppVeyor matrix validates pull requests against both the Debug and Release configurations. If a user working with a Release build locally saw an error appear on GitHub, it would be clear from the build report that it only occurred in a Debug configuration and they could switch to it locally to reproduce it.

[sharwell]

Tagging @vancem for final acceptance.

[vancem]

LGTM