ci: speed up loom tests

[carllerche]

The thread pool loom tests can take 6 hours to run. This is hitting Github Action's upper bound (6 hours).

#5705 switched max preemptions to 1.

I'm not 100% sure what we can do to speed up the tests. At the very least, we could investigate using a rayon-based loom scheduler to add some concurrency. There also is the option of adding 1.5 max-preemption, which would be implemented by randomly picking 1 or 2 max preemptions per branch exploration.