Minimize Timing Issue in test WALTrashCleanupOnOpen (#7796)

Summary:
We saw DBWALTestWithParam/DBWALTestWithParam.WALTrashCleanupOnOpen sometimes fail with:

db/db_sst_test.cc:575: Failure
Expected: (trash_log_count) >= (1), actual: 0 vs 1

The suspicious is that delete scheduling actually deleted all trash files based on rate, but it is not expected. This can be reproduced if we manually add sleep after DB is closed for serveral seconds. Minimize its chance by setting the delete rate to be lowest possible.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7796

Test Plan: The test doesn't fail with the manual sleeping anymore

Reviewed By: anand1976

Differential Revision: D25675000

fbshipit-source-id: a39fd05e1a83719c41014e48843792e752368e22

db/db_sst_test.cc

@@ -544,6 +544,11 @@ TEST_P(DBWALTestWithParam, WALTrashCleanupOnOpen) {
 
   // Create 4 files in L0
   for (char v = 'a'; v <= 'd'; v++) {
+    if (v == 'd') {
+      // Maximize the change that the last log file will be preserved in trash
+      // before restarting the DB.
+      options.sst_file_manager->SetDeleteRateBytesPerSecond(1);
+    }
     ASSERT_OK(Put("Key2", DummyString(1024, v)));
     ASSERT_OK(Put("Key3", DummyString(1024, v)));
     ASSERT_OK(Put("Key4", DummyString(1024, v)));