Run tests compiled with the previous version of Qt with the newest libraries in CI

The test coverage for binary compatibility is not without gaps at the moment.
There are some tests in qtqa that compare the exported symbols.

The test coverage could be increased by storing unit tests compiled with e.g. 6.7.0 in a repository and running them with the newest version of the library on a regular basis. If they run without crashing, it would be a strong indication that the binary compatibility is intact.
In fact, most tests should be successful and the rest could be blacklisted.

This could be included in CI and run on a weekly basis.