I've already succeeded in making both compile/build and unit test run in one job with the studio3 PR builder. We should do the same for the normal studio3 job. That way we can eliminate one job; reduce disk usage; speed up the combined time (at least partially); avoid having to use the hack postbuild script to force build status on the core build from the test build.
Only wrinkle is dealing with retaining artifacts. I think we can just specify to retain only the last build's artifacts to save on disk space and still get the time/test result trends (by keeping lots of builds).