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.
Built from https://develop.svn.wordpress.org/trunk@30283


git-svn-id: http://core.svn.wordpress.org/trunk@30283 1a063a9b-81f0-0310-95a4-ce76da25c4cd