OneBuild Invoke-UnitTests task should support other unit test frameworks

[lholman]

Currently OneBuild only supports execution of NUnit unit tests, which other unit test frameworks should we be supporting? Initial thoughts are XUnit and MSTest, however MSTest usually has a hard dependency on Visual Studio being installed on a build agent, which I suppose isn't the concern of OneBuild, rather a dependency for the build to succeed. Regardless, let's use this issue to capture interest in support of additional unit test frameworks.