Implement test source-only branch test sync and build on VMR-full PRs

[mmitche]

When a PR is made to the VMR (in full VMR mode), the VMR will run PR tests. This sync process should also sync a test source-only VMR branch (applying the cloaking process) and run the source-only tests.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[mmitche]

Going with a different solution in #4088