Improvements to encoded character formatting tests.

In `sanitize_title_with_dashes()` and `sanitize_user()` tests, we break large
test methods into smaller ones in order to isolate those that actually describe
the bug being reported in ticket #10823. The unit tests that are continuing to
fail have been attached as a patch to that ticket.

See #30284.

git-svn-id: https://develop.svn.wordpress.org/trunk@30515 602fd350-edb4-49c9-b593-d223f7449a82