Reduce #iterations to shorten execution time. (#4123)

Summary:
Reduce #iterations from 5000 to 1000 so that
`ExternalSSTFileTest.CompactDuringAddFileRandom` can finish faster.
On the one hand, 5000 iterations does not seem to improve the quality of unit
test in comparison with 1000. On the other hand, long running tests should belong to stress tests.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4123

Differential Revision: D8822514

Pulled By: riversand963

fbshipit-source-id: 0f439b8d5ccd9a4aed84638f8bac16382de17245

db/external_sst_file_test.cc

@@ -1194,8 +1194,9 @@ TEST_F(ExternalSSTFileTest, CompactDuringAddFileRandom) {
     ASSERT_OK(GenerateAndAddExternalFile(options, file_keys, range_id));
   };
 
+  const int num_of_ranges = 1000;
   std::vector<port::Thread> threads;
-  while (range_id < 5000) {
+  while (range_id < num_of_ranges) {
     int range_start = range_id * 10;
     int range_end = range_start + 10;
 
@@ -1220,7 +1221,7 @@ TEST_F(ExternalSSTFileTest, CompactDuringAddFileRandom) {
     range_id++;
   }
 
-  for (int rid = 0; rid < 5000; rid++) {
+  for (int rid = 0; rid < num_of_ranges; rid++) {
     int range_start = rid * 10;
     int range_end = range_start + 10;