Fix a harmless data race affecting two test cases (#8055)

Summary:
`DBTest.GetLiveBlobFiles` and `ObsoleteFilesTest.BlobFiles` both modify the
current `Version` in their setup phase, implicitly assuming that no other
threads would touch the `Version` while this is happening. The periodic
stats dumper thread violates this assumption; the patch fixes this by
disabling it in the affected test cases. (Note: the data race is
harmless in the sense that it only affects test code.)

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8055

Test Plan:
```
COMPILE_WITH_TSAN=1 make db_test -j24
gtest-parallel --repeat=10000 ./db_test --gtest_filter="*GetLiveBlobFiles"
COMPILE_WITH_TSAN=1 make obsolete_files_test -j24
gtest-parallel --repeat=10000 ./obsolete_files_test --gtest_filter="*BlobFiles"
```

Reviewed By: riversand963

Differential Revision: D27022715

Pulled By: ltamasi

fbshipit-source-id: b6cc77ed63d8bc1cbe0603522ff1a572182fc9ab

db/db_test.cc

@@ -2332,6 +2332,13 @@ TEST_F(DBTest, ReadonlyDBGetLiveManifestSize) {
 }
 
 TEST_F(DBTest, GetLiveBlobFiles) {
+  // Note: the following prevents an otherwise harmless data race between the
+  // test setup code (AddBlobFile) below and the periodic stat dumping thread.
+  Options options = CurrentOptions();
+  options.stats_dump_period_sec = 0;
+
+  Reopen(options);
+
   VersionSet* const versions = dbfull()->TEST_GetVersionSet();
   assert(versions);
   assert(versions->GetColumnFamilySet());

db/obsolete_files_test.cc

@@ -94,6 +94,12 @@ class ObsoleteFilesTest : public DBTestBase {
     options.WAL_ttl_seconds = 300;     // Used to test log files
     options.WAL_size_limit_MB = 1024;  // Used to test log files
     options.wal_dir = wal_dir_;
+
+    // Note: the following prevents an otherwise harmless data race between the
+    // test setup code (AddBlobFile) in ObsoleteFilesTest.BlobFiles and the
+    // periodic stat dumping thread.
+    options.stats_dump_period_sec = 0;
+
     Destroy(options);
     Reopen(options);
   }
@@ -192,6 +198,8 @@ TEST_F(ObsoleteFilesTest, DeleteObsoleteOptionsFile) {
 }
 
 TEST_F(ObsoleteFilesTest, BlobFiles) {
+  ReopenDB();
+
   VersionSet* const versions = dbfull()->TEST_GetVersionSet();
   assert(versions);
   assert(versions->GetColumnFamilySet());