Improve `Tests_Feed_RSS2::test_items()`.

* Better reference to post author, to avoid PHP notices.
* Code styling.
* More reliable checking of tags and categories.

Props kurtpayne.
Fixes #UT32. See #30284.

git-svn-id: https://develop.svn.wordpress.org/trunk@30283 602fd350-edb4-49c9-b593-d223f7449a82