Switch integration testing config to a fixture

The hack of storing the config on the class is left in;
it would be too much work for too little gain at this time.
Integration tests: Port the BeakerLib plugin and log collection to pytest

Move the IPA-specific log collection out of the Beakerlib plugin.
Add the --logfile-dir option to tests and ipa-test-task, so that logs
can be collected even if BeakerLib is not used.