Qt5 CI: handle unstable autotests

We need a method of handling unstable autotests in `make check'.

This may require some changes to testlib, or just to the test scripts.

We need `make check' to be clean - i.e., it gives a zero exit code if all stable tests compiled and passed. There should be a way for `make check' to not fail if an autotest which is known to be unstable fails.

These autotests have currently been disabled, and should be re-enabled when we have a way to handle unstable tests:

• qtqa/tests/guiapplauncher