Blackbox tests fail when the qbs_autotests profile is not set up in the expected manner

When running blackbox tests with an incorrectly set up qbs_autotests profile (for example, missing Qt properties), the tests will fail. It would be helpful if the tests validated that the profile is set up as expected in order to produce a more helpful error message.

Jörg: "We could add checks to tst_blackbox whether the qbs_autotests profile
has Qt properties set. Roughly at the same location where it checks
whether the qbs_autotests profile exists."