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.
Built from https://develop.svn.wordpress.org/trunk@30515


git-svn-id: http://core.svn.wordpress.org/trunk@30504 1a063a9b-81f0-0310-95a4-ce76da25c4cd