Reduce execution time of a test. (#4127)

Summary:
Reduce the number of key ranges in `ExternalSSTFileTest.OverlappingRanges` so
that the test completes in shorter time to avoid timeouts.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4127

Differential Revision: D8827851

Pulled By: riversand963

fbshipit-source-id: a16387b0cc92a7c872b1c50f0cfbadc463afc9db

db/external_sst_file_test.cc

@@ -925,7 +925,7 @@ TEST_F(ExternalSSTFileTest, OverlappingRanges) {
 
     printf("Option config = %d\n", option_config_);
     std::vector<std::pair<int, int>> key_ranges;
-    for (int i = 0; i < 500; i++) {
+    for (int i = 0; i < 100; i++) {
       int range_start = rnd.Uniform(20000);
       int keys_per_range = 10 + rnd.Uniform(41);