tests: enable frontend-multiprocessing parametrization

[dtrifiro]

Revert "tests: temporarily disable frontend multiprocessing parametrization"

[dtrifiro]

Tests pass with parametrization enabled, but pytest exists with error:

================== 13 passed, 2 skipped, 4 warnings in 25.39s ==================
--- Logging error ---
Fatal Python error: _enter_buffered_busy: could not acquire lock for <_io.BufferedWriter name='<stderr>'> at interpreter shutdown, possibly due to daemon threads
Python runtime state: finalizing (tstate=0x00007f8973376358)

Current thread 0x00007f8973439b80 (most recent call first):
  <no Python frame>

This is probably related to the way we spin up vLLM in a loop in a separate thread in the _servers fixture.