Clean up 'post-thumbnail' theme support unit tests.

* Separate into smaller test methods.
* Remove incorrect tests. A number of assertions in the existing test have always incorrectly described the behavior of `current_theme_supports( 'post-thumbnails' )`, but no one ever noticed because the tests had been designed to bail when `_wp_render_title_tag()` did not exist. The failures finally became visible when that function was introduced in [30074].

See #18548.

git-svn-id: https://develop.svn.wordpress.org/trunk@30148 602fd350-edb4-49c9-b593-d223f7449a82