Add support for running tests with Qt5 and/or Qt6

Enable [5,6] for use-chrono-in-qtimer, as a first example where it just works, and in lambda-unique-connection where Qt6 changed things and this includes a fix.

clang-apply-replacements runs on the output of the last Qt version being tested, since they overwrite the same yaml files. Doing that per Qt version would require further refactoring.