Add basic ci

See also issue #4 (closed) this MR adds a basic CI job which runs on any push, to check that autotests still pass.

Some points are worth noting:

• No coverage yet, also no idea how to do that with CTest...
• Tests are invoked through ctest directly, and output is in native CTest XML format. I.e. no pretty test results in Gitlab yet, that would require JUnit XML type output. Perversely, the test executables invoked by CTest do have native xUnit output logging support, but how to enable that from CTest I have no idea...
• Yes: I could have scripted this out verbatim in YAML. No: I don't think that would be a good idea.
• Despite being only a rough outline so far, it did already catch 3 issues:
  1. A failing test for the unsigned long long validator (counter): French is weird, at least the French QLocale definitely is inconsistent.
  2. Some incorrect/duplicate find_package() delcarations: Kirigami is not optional, for instance, and Qt5::Test was mentioned twice, using potentially different version requirements.
  3. It seems we did not, in fact, honour ENABLE_TESTING and some other CTest related weirdness. Hopefully, this should now be fixed.