Revert changes which broke reuse of workers

[giordano]

CC @jishnub.

[giordano]

I added a test to prevent this regression again. I verified that running this test on main (7f27702, but also on 16f47d4), it'd fail:

reuse of workers: Test Failed at /Users/mose/.julia/dev/ParallelTestRunner/test/runtests.jl:401
  Expression: ParallelTestRunner.ID_COUNTER[] == old_id_counter + njobs
   Evaluated: 30 == 25