tst_qmlformat::testExample issues: slow + triggers warnings

testExample tries to run over all test and example files. As such, it's not really a unit test, but rather an integration test.

It tends to be rather slow. While this might be an opportunity for profiling, we probably shouldn't run this test as an auto test, but restrict it to nightly/manual.

Lastly, the test also seems to trigger some warnings, which are however not picked up.