Allow compaction to finish in TEST_CompactRange

Waits for a current transaction to finish in TEST_CompactRange when the
`background_work_finished_signal_` signal may be emitted due to an error. This is a prospective fix for flaky crashes observed in b/351680196#comment5.

PiperOrigin-RevId: 663457574

db/db_impl.cc

@@ -629,6 +629,11 @@ void DBImpl::TEST_CompactRange(int level, const Slice* begin,
       background_work_finished_signal_.Wait();
     }
   }
+  // Finish current background compaction in the case where
+  // `background_work_finished_signal_` was signalled due to an error.
+  while (background_compaction_scheduled_) {
+    background_work_finished_signal_.Wait();
+  }
   if (manual_compaction_ == &manual) {
     // Cancel my manual compaction since we aborted early for some reason.
     manual_compaction_ = nullptr;