FileLocationUtilities.GetFileDistributedWithApplication does not work when running tests under Visual Studio Test Platform

[tombogle]

If we can figure out an appropriate way in ReflectionHelper to detect that we're running under Visual Studio Test Platform, it should be possible to change the logic to return the executing assembly. Then maybe in FileLocationUtilities.GetFileDistributedWithApplication, we could note that we're running from inside a package location and search within the package contents to find the desired "distfile."
This issue came up because required distfiles were added to GlyssenEngine (for use in Vessel), but when running unit tests using microsoft.testplatform.testhost, it works very differently from nunit, and it cannot locate those files.