Remove timing from source compilation test

[DanielThomas]

Summary

• replace the timing-based concurrency assertion with a functional compileAll integration test
• compile two independent valid modules and verify both outputs
• remove the diagnostic callback barrier that raced with compileAll cancelling peers after the first expected compilation failure

The race appeared in the first v0.16.6 release attempt: https://github.com/Netflix/jig/actions/runs/36107353801/attempts/1

Testing

• ja test --all (555 tests)