Change directory where ExternalSSTFileBasicTest runs (#4766)

Summary:
Change the directory where ExternalSSTFileBasicTest* tests run.

**Problem:**
Without this change, I spent considerable time chasing around a non-existent issue as ExternalSSTFileTest.* and ExternalSSTFileBasicTest.* create similar directories.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4766

Differential Revision: D13409384

Pulled By: sagar0

fbshipit-source-id: c33e1f4d505dfa6efbc788d6c57cdb680053ded3

db/external_sst_file_basic_test.cc

@@ -17,7 +17,7 @@ namespace rocksdb {
 class ExternalSSTFileBasicTest : public DBTestBase,
                                  public ::testing::WithParamInterface<bool> {
  public:
-  ExternalSSTFileBasicTest() : DBTestBase("/external_sst_file_test") {
+  ExternalSSTFileBasicTest() : DBTestBase("/external_sst_file_basic_test") {
     sst_files_dir_ = dbname_ + "/sst_files/";
     DestroyAndRecreateExternalSSTFilesDir();
   }