Attach a process-mode counting PMC to the current process, start it,
then detach and release it while it is still loaded on the hardware -
the case that previously leaked the PMC's runcount reference and
wedged pmc_wait_for_pmc_idle() at release. A second case does the same
from a multi-threaded process so the sibling threads' references have
to be drained too.

The tests need an allocatable process-mode counting event and skip
where none is available (hwpmc(4) not loaded, or a VM without a vPMU).

MFC after: 2 weeks
Assisted-by: Claude Code (Fable 5)