Clean up after two test failures in db_basic_test (#7682)

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

db/db_basic_test.cc

@@ -38,7 +38,10 @@ TEST_F(DBBasicTest, OpenWhenOpen) {
   options.env = env_;
   DB* db2 = nullptr;
   Status s = DB::Open(options, dbname_, &db2);
-  ASSERT_NOK(s);
+  ASSERT_NOK(s) << [db2]() {
+    delete db2;
+    return "db2 open: ok";
+  }();
   ASSERT_EQ(Status::Code::kIOError, s.code());
   ASSERT_EQ(Status::SubCode::kNone, s.subcode());
   ASSERT_TRUE(strstr(s.getState(), "lock ") != nullptr);
@@ -402,13 +405,16 @@ TEST_F(DBBasicTest, GetSnapshot) {
 
 TEST_F(DBBasicTest, CheckLock) {
   do {
-    DB* localdb;
+    DB* localdb = nullptr;
     Options options = CurrentOptions();
     ASSERT_OK(TryReopen(options));
 
     // second open should fail
     Status s = DB::Open(options, dbname_, &localdb);
-    ASSERT_NOK(s);
+    ASSERT_NOK(s) << [localdb]() {
+      delete localdb;
+      return "localdb open: ok";
+    }();
 #ifdef OS_LINUX
     ASSERT_TRUE(s.ToString().find("lock ") != std::string::npos);
 #endif  // OS_LINUX