Clear Posts otherwise select_one might fail.

Sometimes `select_one` in this test will return other posts loaded from the fixtures. If we want to make sure `select_one` is correct without the `where` condition, we will have to make sure the record is the only one.

cc @senny aren't all fixtures unloaded before each test, or am I missing anything? I've been digging around to find the cause but all attempts were futile. I'm not sure if this workaround is the best approach.