Repair the `seems_utf8()` tests that use Big5 encoding.

* Provide sample data that is actually encoded in Big5.
* Do some actual assertions against that data.

Props akumria.
Fixes #30455.
Built from https://develop.svn.wordpress.org/trunk@30525


git-svn-id: http://core.svn.wordpress.org/trunk@30514 1a063a9b-81f0-0310-95a4-ce76da25c4cd