Reduce the number of iterations in DBTest.FileCreationRandomFailure (#7481)

Summary:
`DBTest.FileCreationRandomFailure` frequently times out during our
continuous test runs. (It's a case of "stress test posing as unit test.")
The patch reduces the number of iterations to avoid this. Note that
the lower numbers are still sufficient to trigger both flushes and
compactions, so test coverage is still the same.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7481

Test Plan: `make check`

Reviewed By: riversand963

Differential Revision: D24034712

Pulled By: ltamasi

fbshipit-source-id: 8731a9446e5a121a1041b00f0df473b9f714935a

db/db_test.cc

@@ -5156,12 +5156,13 @@ TEST_F(DBTest, FileCreationRandomFailure) {
   DestroyAndReopen(options);
   Random rnd(301);
 
-  const int kCDTKeysPerBuffer = 4;
-  const int kTestSize = kCDTKeysPerBuffer * 4096;
-  const int kTotalIteration = 100;
+  constexpr int kCDTKeysPerBuffer = 4;
+  constexpr int kTestSize = kCDTKeysPerBuffer * 4096;
+  constexpr int kTotalIteration = 20;
   // the second half of the test involves in random failure
   // of file creation.
-  const int kRandomFailureTest = kTotalIteration / 2;
+  constexpr int kRandomFailureTest = kTotalIteration / 2;
+
   std::vector<std::string> values;
   for (int i = 0; i < kTestSize; ++i) {
     values.push_back("NOT_FOUND");