Ensure themes don't have to call add_image_size() to be used in unit tests.

props mboynes.
fixes #29924.

git-svn-id: https://develop.svn.wordpress.org/trunk@29874 602fd350-edb4-49c9-b593-d223f7449a82
Improve unit test coverage for wp_update_term().

See #5809, #22023.

git-svn-id: https://develop.svn.wordpress.org/trunk@29875 602fd350-edb4-49c9-b593-d223f7449a82