Make initalization of QList<QUuid> explicit, to fix building with Qt6

[daschuer]

This should fix

/__w/mixxx/mixxx/src/test/musicbrainzrecordingstasktest.cpp: In constructor ‘MusicBrainzRecordingsTaskTest::MusicBrainzRecordingsTaskTest()’:
Error: /__w/mixxx/mixxx/src/test/musicbrainzrecordingstasktest.cpp:19:55: error: no match for ‘operator=’ (operand types are ‘QList<QUuid>’ and ‘<brace-enclosed initializer list>’)
   19 |                 "5f6340ae-9cab-4f00-83d5-7ad00ac35f5b"};
      |                                                       ^

https://github.com/mixxxdj/mixxx/actions/runs/3452375864/jobs/5762241905

[ronso0]

LGTM, thanks.