Since [1] some timings seem to have changed. This caused the unit tests
to fail intermittently, from my testings especially on Ubuntu systems
(much less often on e.g. Arch).
This commit adds the `dockergen_wait_for_event` helper-function to try
and wait for the configuration to be generated by docker-gen before
continuing on with the actual tests themselves.
Additionally, at the end of every test file, all containers spun up by
the bats-tests will be stopped. This required adding the `bats-type`
label to every container started during the bats-tests.
The stopping of the containers reduces the amount of events docker-gen
has to process, thus resulting in lower wait times for the generation to
happen.
[1]: 50435652b1
