Fix flaky test DBTest2.VariousFileTemperatures (#12974)

Summary:
... apparently due to potentially not purging obsolete files after CompactRange

Example: https://github.com/facebook/rocksdb/actions/runs/10564621261/job/29267393711?pr=12959

Pull Request resolved: #12974

Test Plan: reproduced failure with USE_CLANG=1 COERCE_CONTEXT_SWITCH=1, now fixed

Reviewed By: cbi42

Differential Revision: D61812600

Pulled By: pdillinger

fbshipit-source-id: d4b23e1a179bb8ec39875ed7a8ce1649fa3344bd

Author: pdillinger

db/db_test2.cc

@@ -6739,6 +6739,7 @@ TEST_F(DBTest2, VariousFileTemperatures) {
                 TCM({{options.default_write_temperature, 2}}));
 
       ASSERT_OK(db_->CompactRange({}, nullptr, nullptr));
+      ASSERT_OK(dbfull()->TEST_WaitForBackgroundWork());
 
       ASSERT_EQ(test_fs->CountCurrentSstFilesByTemp(),
                 TCM({{options.last_level_temperature, 1}}));