Speed up interactivetests

[Kha]

I finally had samply look into why these tests can take 10s or more and my initial guess only made second place:

1. most expensive: even after shutdown, the watchdog process waits on the .ilean loader task because we're not using Process.exit like in the file worker
2. We recompile (in-memory) run.lean for every test; we should precompile it in the overall build
3. even with Process.exit, somehow the watchdog main thread waits ~200ms for the .ilean loader (just a time slice??)