Fix rare failure in DBBlockCacheTypeTest.Uncache

Summary: Following up on facebook#12748 after seeing recurrence in
https://github.com/facebook/rocksdb/actions/runs/9522985253/job/26253605587?pr=12774

Test Plan: Was able to reproduce failure and verify fix this time
using COERCE_CONTEXT_SWITCH=1 :)

db/db_block_cache_test.cc

@@ -1409,6 +1409,7 @@ TEST_P(DBBlockCacheTypeTest, Uncache) {
 
       // Combine into one file, making the originals obsolete
       ASSERT_OK(db_->CompactRange({}, nullptr, nullptr));
+      ASSERT_OK(dbfull()->TEST_WaitForBackgroundWork());
 
       ASSERT_EQ(1, NumTableFilesAtLevel(1));