Replace the obsolete mathtext reference test with a public-behavior test

[cvanelteren]

ultraplot/tests/test_fonts.py contains a skipped test_replacement() placeholder that explicitly relies on private MathTextParser class attributes. That makes it a poor long-term regression test.

This issue is intentionally scoped for newcomers to the project. It is low complexity, limited to the fonts test module, and should help someone get familiar with the repo's testing style and the mathtext font integration code.

Suggested scope:

• replace the skipped placeholder with a test that checks supported public behavior
• avoid depending on private Matplotlib class attributes or implementation details
• if the existing font tests already cover the behavior, remove the obsolete placeholder instead of keeping dead weight

Acceptance criteria:

• the skipped placeholder is removed or replaced
• the resulting test does not depend on private Matplotlib internals
• the fonts test suite still passes cleanly