Fix flaky WALTrashCleanupOnOpen (#8917)

Summary:
Test did not consider that slower deletion rate only kicks in
after a file is deleted

Fixes https://github.com/facebook/rocksdb/issues/7546

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8917

Test Plan:
no longer reproduces using

    buck test mode/dev //internal_repo_rocksdb/repo:db_sst_test -- --exact 'internal_repo_rocksdb/repo:db_sst_test - DBWALTestWithParam/DBWALTestWithParam.WALTrashCleanupOnOpen/0' --jobs 40 --stress-runs 600 --record-results

Reviewed By: siying

Differential Revision: D30949127

Pulled By: pdillinger

fbshipit-source-id: 5d0607f8f548071b07410fe8f532b4618cd225e5

db/db_sst_test.cc

@@ -809,9 +809,11 @@ TEST_P(DBWALTestWithParam, WALTrashCleanupOnOpen) {
 
   // Create 4 files in L0
   for (char v = 'a'; v <= 'd'; v++) {
-    if (v == 'd') {
+    if (v == 'c') {
       // Maximize the change that the last log file will be preserved in trash
       // before restarting the DB.
+      // We have to set this on the 2nd to last file for it to delay deletion
+      // on the last file. (Quirk of DeleteScheduler::BackgroundEmptyTrash())
       options.sst_file_manager->SetDeleteRateBytesPerSecond(1);
     }
     ASSERT_OK(Put("Key2", DummyString(1024, v)));