Change coreclr YML to build tests and product in parallel

[jashook]

Initial cost estimate: 2 weeks
Initial contacts: @trylek, @jkoritzinsky, @safern

This will allow us to cut the end to end time of the build from ~2 hours to ~1 hour.

[jkoritzinsky]

Due to #34664 removing the separate CoreLib job, the path forward on this issue looks to be to build CoreLib during the CoreCLR test build job for tests that directly reference System.Private.CoreLib.