This CL ensures that CompactRange doesn't get stuck on waiting on background_work_finished_signal_ during shutdown.
While background_work_finished_signal_ may be in fact signaled, CompactMemTable() will never run, not giving opportunity for workers blocked in TEST_CompactMemTable to exit the loop.
We suspect this contributes to b/370844779; while the hang was mitigated
by limiting concurrency to 1 worker (which avoids filling the thread pool with
blocked workers), this issue can still happen and will block 1 worker during
shutdown.
PiperOrigin-RevId: 879794057