Avoid skipping a test in db_wal_test (#7628)

Summary:
Recent test report shows that some tests have been skipped.

For DBWALTest that inherits from DBTestBase, the following will always be
true, since `env_` is an instance of `SpecialEnv`, not `Env::Default()`. Thus the test
will always be skipped.

```
if (options.env != Env::Default()) {
  ROCKSDB_GTEST_SKIP("Test requires default environment");
  return;
}
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7628

Test Plan:
./db_wal_test --gtest_filter=DBWALTest.TruncateLastLogAfterRecoverWithoutFlush
MEM_ENV=1 ./db_wal_test --gtest_filter=DBWALTest.TruncateLastLogAfterRecoverWithoutFlush
make check

Reviewed By: jay-zhuang

Differential Revision: D24693006

Pulled By: riversand963

fbshipit-source-id: 7f2a772492a0f11bff17bbf5e9f493e9e9a1c125

db/db_wal_test.cc

@@ -1696,9 +1696,10 @@ TEST_F(DBWALTest, RestoreTotalLogSizeAfterRecoverWithoutFlush) {
 TEST_F(DBWALTest, TruncateLastLogAfterRecoverWithoutFlush) {
   constexpr size_t kKB = 1024;
   Options options = CurrentOptions();
+  options.env = env_;
   options.avoid_flush_during_recovery = true;
-  if (options.env != Env::Default()) {
-    ROCKSDB_GTEST_SKIP("Test requires default environment");
+  if (mem_env_) {
+    ROCKSDB_GTEST_SKIP("Test requires non-mem environment");
     return;
   }
   // Test fallocate support of running file system.