Add tests for categories

* There was a duplicated test for `category` (https://github.com/jekyll/jekyll/blob/master/features/post_data.feature#L122-131 and https://github.com/jekyll/jekyll/blob/master/features/post_data.feature#L144-153). Changed the latter to test `categories`.
* Add missed test when category is in a folder and has category or categories in YAML.
* Add tests for duplicated categories.