Avoid skipping a test in db_test2 (facebook#7629)

Summary:
Test report shows that this test has been skipped recently due to
a condition that will never meet. `env_` is not equal to
`Env::Default()` for DBTest2 that inherits from DBTestBase.

Pull Request resolved: facebook#7629

Test Plan:
make check
./db_test2 --gtest_filter=DBTest2.PinnableSliceAndMmapReads

Reviewed By: jay-zhuang

Differential Revision: D24693317

Pulled By: riversand963

fbshipit-source-id: b1bbd5c1e05a6fa57c1de0d74462b69e3c2d5215

db/db_test2.cc

@@ -4163,7 +4163,7 @@ TEST_F(DBTest2, TraceWithFilter) {
 TEST_F(DBTest2, PinnableSliceAndMmapReads) {
   Options options = CurrentOptions();
   options.env = env_;
-  if (options.env != Env::Default()) {
+  if (!IsMemoryMappedAccessSupported()) {
     ROCKSDB_GTEST_SKIP("Test requires default environment");
     return;
   }