Summary:
In db_basic_test.cc, there are two tests that rely on the underlying
system's `LockFile` support to function correctly:
DBBasicTest.OpenWhenOpen and DBBasicTest.CheckLock. In both tests,
re-opening a db using `DB::Open` is expected to fail because the second
open cannot lock the LOCK file. Some distributed file systems, e.g. HDFS
do not support the POSIX-style file lock. Therefore, these unit tests will cause
assertion failure and the second `Open` will create a db instance.
Currently, these db instances are not closed after the assertion
failure. Since these db instances are registered with some process-wide, static
data structures, e.g. `PeriodicWorkScheduler::Default()`, they can still be
accessed after the unit tests. However, the `Env` object created for this db
instance is destroyed when the test finishes in `~DBTestBase()`. Consequently,
it causes illegal memory access.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7682

Test Plan:
Run the following on a distrubited file system:
```
make check
```

Reviewed By: anand1976

Differential Revision: D25004215

Pulled By: riversand963

fbshipit-source-id: f4327d7716c0e72b13bb43737ec9a5d156da4d52