Only last mp3 on ci

Hello,

this is my first contribution to any pinephone related project (and my second to any OpenSource project), so I am sorry if I have not done anything as expected (but please tell me).

Anyways, hope my solution is not too hacky. Don't mind the critics (and hopefully can learn from them :)).

I got a warning for the second test where all urls are checked except for generate_mp3:

~/contrib/cast.postmarketos.org/tests/test_freeze.py:40: MissingURLGeneratorWarning: Nothing frozen for endpoints generate_mp3, legacyatom, atom. Did you forget a URL generator?

Is that a problem that legacyatom and atom are missing?

Fixes #7 (closed)