Migrate to VSTest for all Unit Tests

[michaelgsharp]

Based on the conversation in #5583 and this comment, we need to migrate all our unit testing over to VSTest to make debugging easier. This issue is being created to track that so we don't lose it.

[michaelgsharp]

@ViktorHofer I set the flag in Versions.props as suggested. It does switch to using VSTest, but even when passing /p:TargetArchitecture=x86 /p:TestArchitectures=x86 it still launches a 64 bit version of VSTest. This is causing test failures. Do you know how to get it to run the x86 version of vstest?