Deflake write-prepared and write-unprepared tests (#8696)

Deflake write-prepared and write-unprepared tests (#8696)

Summary:
The `JobContext::job_snapshot` referenced DB state but could
have been deleted by a BG thread after the signal/unlock allowing
shutdown to proceed. Then we would see an error like this (valgrind):

```
==354104== Thread 2:
==354104== Invalid read of size 8
==354104==    at 0x694C4D: rocksdb::ManagedSnapshot::~ManagedSnapshot() (snapshot_impl.cc:20)
==354104==    by 0x58F5BA: operator() (unique_ptr.h:81)
==354104==    by 0x58F5BA: operator() (unique_ptr.h:75)
==354104==    by 0x58F5BA: ~unique_ptr (unique_ptr.h:292)
==354104==    by 0x58F5BA: rocksdb::JobContext::~JobContext() (job_context.h:221)
==354104==    by 0x5F155E: rocksdb::DBImpl::BackgroundCallCompaction(rocksdb::DBImpl::PrepickedCompaction*, rocksdb::Env::Priority) (db_impl_compaction_flush.cc:2696)
==354104==    by 0x5F1BC2: rocksdb::DBImpl::BGWorkCompaction(void*) (db_impl_compaction_flush.cc:2468)
==354104==    by 0x83707A: operator() (std_function.h:688)
==354104==    by 0x83707A: rocksdb::ThreadPoolImpl::Impl::BGThread(unsigned long) (threadpool_imp.cc:266)
==354104==    by 0x8373ED: rocksdb::ThreadPoolImpl::Impl::BGThreadWrapper(void*) (threadpool_imp.cc:307)
==354104==    by 0x492A800: execute_native_thread_routine (in /usr/local/fbcode/platform009/lib/libstdc++.so.6.0.28)
==354104==    by 0x4A5020B: start_thread (in /usr/local/fbcode/platform009/lib/libpthread-2.30.so)
==354104==    by 0x4CF281E: clone (in /usr/local/fbcode/platform009/lib/libc-2.30.so)
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8696

Test Plan: unable to repro

Reviewed By: pdillinger

Differential Revision: D30505277

Pulled By: ajkr

fbshipit-source-id: 5a99f34137cd14d06b0f624add6d37a70a61135d