Do not use ASSERT_OK in child threads in ExternalSstFileTest.PickedLevelBug (#7754)

Summary:
`googletest` uses exceptions to communicate assertion failures when
`GTEST_THROW_ON_FAILURE` is set, which does not go well with
`std::thread`s, since an exception escaping the top-level function of an
`std::thread` object or an `std::thread` getting destroyed without
having been `join`ed or `detach`ed first results in a call to
`std::terminate`. The patch fixes this by moving the `Status` assertions
of background operations in `ExternalSstFileTest.PickedLevelBug` to the
main thread.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7754

Test Plan: `make check`

Reviewed By: riversand963

Differential Revision: D25383808

Pulled By: ltamasi

fbshipit-source-id: 32fb2721e5169ec898d218900bc0d83eead45d03

db/external_sst_file_test.cc

@@ -1345,15 +1345,18 @@ TEST_F(ExternalSSTFileTest, PickedLevelBug) {
   ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->EnableProcessing();
 
   // While writing the MANIFEST start a thread that will ask for compaction
+  Status bg_compact_status;
   ROCKSDB_NAMESPACE::port::Thread bg_compact([&]() {
-    ASSERT_OK(db_->CompactRange(CompactRangeOptions(), nullptr, nullptr));
+    bg_compact_status =
+        db_->CompactRange(CompactRangeOptions(), nullptr, nullptr);
   });
   TEST_SYNC_POINT("ExternalSSTFileTest::PickedLevelBug:2");
 
   // Start a thread that will ingest a new file
+  Status bg_addfile_status;
   ROCKSDB_NAMESPACE::port::Thread bg_addfile([&]() {
     file_keys = {1, 2, 3};
-    ASSERT_OK(GenerateAndAddExternalFile(options, file_keys, 1));
+    bg_addfile_status = GenerateAndAddExternalFile(options, file_keys, 1);
   });
 
   // Wait for AddFile to start picking levels and writing MANIFEST
@@ -1374,6 +1377,9 @@ TEST_F(ExternalSSTFileTest, PickedLevelBug) {
   bg_addfile.join();
   bg_compact.join();
 
+  ASSERT_OK(bg_addfile_status);
+  ASSERT_OK(bg_compact_status);
+
   dbfull()->TEST_WaitForCompact();
 
   int total_keys = 0;