Attempt to deflake DBWALTestWithEnrichedEnv.SkipDeletedWALs (#11537)

Summary:
Calling `Flush` (even with `wait==true`) does not guarantee that obsolete WAL files are physically deleted before the call returns. The patch attempts to fix the resulting flakiness by using `SyncPoint`s to make sure `PurgeObsoleteFiles` finishes before checking for WAL deletions.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11537

Test Plan:
```
gtest-parallel --repeat=1000 ./db_wal_test --gtest_filter="*SkipDeletedWALs*"
```

Reviewed By: pdillinger

Differential Revision: D46736050

Pulled By: ltamasi

fbshipit-source-id: 47a931b7a3a03ef681fbf4adb5a0b223d452703e

db/db_wal_test.cc

@@ -146,6 +146,11 @@ TEST_F(DBWALTestWithEnrichedEnv, SkipDeletedWALs) {
   options.write_buffer_size = 128;
   Reopen(options);
 
+  SyncPoint::GetInstance()->LoadDependency(
+      {{"DBImpl::PurgeObsoleteFiles:End",
+        "DBWALTestWithEnrichedEnv.SkipDeletedWALs:AfterFlush"}});
+  SyncPoint::GetInstance()->EnableProcessing();
+
   WriteOptions writeOpt = WriteOptions();
   for (int i = 0; i < 128 * 5; i++) {
     ASSERT_OK(dbfull()->Put(writeOpt, "foo", "v1"));
@@ -154,6 +159,8 @@ TEST_F(DBWALTestWithEnrichedEnv, SkipDeletedWALs) {
   fo.wait = true;
   ASSERT_OK(db_->Flush(fo));
 
+  TEST_SYNC_POINT("DBWALTestWithEnrichedEnv.SkipDeletedWALs:AfterFlush");
+
   // some wals are deleted
   ASSERT_NE(0, enriched_env_->deleted_wal_cnt);
   // but not the first one
@@ -164,6 +171,8 @@ TEST_F(DBWALTestWithEnrichedEnv, SkipDeletedWALs) {
   Reopen(options);
   ASSERT_FALSE(enriched_env_->deleted_wal_reopened);
   ASSERT_FALSE(enriched_env_->gap_in_wals);
+
+  SyncPoint::GetInstance()->DisableProcessing();
 }
 
 TEST_F(DBWALTest, WAL) {