Improve the robustness of ThreadStatusSingleCompaction

Summary:
Improve the robustness of ThreadStatusSingleCompaction
by ensuring the number of files flushed in the test.

Test Plan:
export ROCKSDB_TESTS=ThreadStatus
./db_test

Reviewers: sdong, igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D35019

db/db_test.cc

@@ -10176,10 +10176,13 @@ TEST(DBTest, ThreadStatusSingleCompaction) {
     DestroyAndReopen(options);
 
     Random rnd(301);
-    for (int key = kEntriesPerBuffer * kNumL0Files; key >= 0; --key) {
-      ASSERT_OK(Put(ToString(key), RandomString(&rnd, kTestValueSize)));
+    for (int file = 0; file < kNumL0Files; ++file) {
+      for (int key = 0; key < kEntriesPerBuffer; ++key) {
+        ASSERT_OK(Put(ToString(key + file * kEntriesPerBuffer),
+                      RandomString(&rnd, kTestValueSize)));
+      }
+      Flush();
     }
-    Flush();
     ASSERT_GE(NumTableFilesAtLevel(0),
               options.level0_file_num_compaction_trigger);