Fix TEST_CompactMemTable deadlock

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

db/db_impl.cc

@@ -646,7 +646,8 @@ Status DBImpl::TEST_CompactMemTable() {
   if (s.ok()) {
     // Wait until the compaction completes
     MutexLock l(&mutex_);
-    while (imm_ != nullptr && bg_error_.ok()) {
+    while (imm_ != nullptr && bg_error_.ok() &&
+           !shutting_down_.load(std::memory_order_acquire)) {
       background_work_finished_signal_.Wait();
     }
     if (imm_ != nullptr) {