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.

git-svn-id: https://develop.svn.wordpress.org/trunk@30525 602fd350-edb4-49c9-b593-d223f7449a82