Deflake RateLimiting/BackupEngineRateLimitingTestWithParam (#10271)

Summary:
We saw flakes with the following failure:

```
[ RUN      ] RateLimiting/BackupEngineRateLimitingTestWithParam.RateLimiting/1
utilities/backup/backup_engine_test.cc:2667: Failure
Expected: (restore_time) > (0.8 * rate_limited_restore_time), actual: 48269 vs 60470.4
terminate called after throwing an instance of 'testing::internal::GoogleTestFailureException'
what():  utilities/backup/backup_engine_test.cc:2667: Failure
Expected: (restore_time) > (0.8 * rate_limited_restore_time), actual: 48269 vs 60470.4
Received signal 6 (Aborted)
t/run-backup_engine_test-RateLimiting-BackupEngineRateLimitingTestWithParam.RateLimiting-1: line 4: 1032887 Aborted                 (core dumped) TEST_TMPDIR=$d ./backup_engine_test --gtest_filter=RateLimiting/BackupEngineRateLimitingTestWithParam.RateLimiting/1
```

Investigation revealed we forgot to use the mock time `SystemClock` for
restore rate limiting. Then the test used wall clock time, which made
the execution of "GenericRateLimiter::Request:PostTimedWait"
non-deterministic as wall clock time might have advanced enough that
waiting was not needed.

This PR changes restore rate limiting to use
mock time, which guarantees we always execute
"GenericRateLimiter::Request:PostTimedWait". Then the assertions that
rely on times recorded inside that callback should be robust.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10271

Test Plan:
Applied the following patch which guaranteed repro before the fix.
Verified the test passes after this PR even with that patch applied.

```
 diff --git a/util/rate_limiter.cc b/util/rate_limiter.cc
index f369e3220..6b3ed82fa 100644
 --- a/util/rate_limiter.cc
+++ b/util/rate_limiter.cc
@@ -158,6 +158,7 @@ void GenericRateLimiter::SetBytesPerSecond(int64_t bytes_per_second) {

 void GenericRateLimiter::Request(int64_t bytes, const Env::IOPriority pri,
                                  Statistics* stats) {
+  usleep(100000);
   assert(bytes <= refill_bytes_per_period_.load(std::memory_order_relaxed));
   bytes = std::max(static_cast<int64_t>(0), bytes);
   TEST_SYNC_POINT("GenericRateLimiter::Request");
```

Reviewed By: hx235

Differential Revision: D37499848

Pulled By: ajkr

fbshipit-source-id: fd790d5a192996be8ba13b656751ccc7d8cb8f6e

utilities/backup/backup_engine_test.cc

@@ -2655,8 +2655,10 @@ TEST_P(BackupEngineRateLimitingTestWithParam, RateLimiting) {
   ASSERT_GT(backup_time, 0.8 * rate_limited_backup_time);
 
   OpenBackupEngine();
-  TEST_SetDefaultRateLimitersClock(backup_engine_.get(),
+  TEST_SetDefaultRateLimitersClock(
-                                   special_env->GetSystemClock());
+      backup_engine_.get(),
+      special_env->GetSystemClock() /* backup_rate_limiter_clock */,
+      special_env->GetSystemClock() /* restore_rate_limiter_clock */);
 
   auto start_restore = special_env->NowMicros();
   ASSERT_OK(backup_engine_->RestoreDBFromLatestBackup(dbname_, dbname_));