Fix a noisy unit test.

Summary: The t/DBTest.DropWrites test still fails under certain gcc version in release unit test.

I unfortunately cannot repro the failure (since the compilers have mapped library which I am not able to map to correctly). I am suspecting the clock skew.

Test Plan: Run make check

Reviewers:

CC: sdong igore

Task ID: #7312624

Blame Rev:

db/db_test.cc

@@ -93,10 +93,11 @@ class AtomicCounter {
     uint64_t start = env_->NowMicros();
     while (count_ < count) {
       uint64_t now = env_->NowMicros();
-      cond_count_.TimedWait(now + /*1s*/ 1 * 000 * 000);
-      if (env_->NowMicros() - start > /*10s*/ 10 * 000 * 000) {
+      const uint64_t elapsed_micros = (now > start) ? now - start : 0;
+      if (elapsed_micros > /*10s*/ 10 * 000 * 000) {
         return false;
       }
+      cond_count_.TimedWait(now + /*1s*/ 1 * 000 * 000);
       if (count_ < count) {
         GTEST_LOG_(WARNING) << "WaitFor is taking more time than usual";
       }