Tests: Print invalid UTF-8 as ASCII to fix hosts test reporting failures.

When serializing test output into XML, invalid UTF-8 bytes lead to a failure to load those test results when they are read. This patch adds code to remap those invalid bytes in an ASCII-readable form, whereas the invalid bytes are separated by parentheses and encoded in their hex form.

This ensures that a proper XML file is generated from the testing results.

Developed in: https://github.com/WordPress/wordpress-develop/pull/11620
Discussed in: https://core.trac.wordpress.org/ticket/31992
Reported in: https://github.com/WordPress/phpunit-test-runner/pull/310

Follow-up to: [62225].

Props agulbra, amykamala, codexdemon, dmsnell, mywp459, rolle.
See #31992.


git-svn-id: https://develop.svn.wordpress.org/trunk@62249 602fd350-edb4-49c9-b593-d223f7449a82