Clean up media that were created during a scenario.

[pfrenssen]

This is a WIP implementation of #103 but unfortunately I am running out of time on my ticket and I can not finish this unless we get a dedicated ticket for this. Here is the code so far. The idea is to not rely on magic for cleaning up media entities created during a scenario, but to keep track of the entities that are created, and clean them up at the end of the test.

[vever001]

Just leaving a small comment here because I remember seeing something similar in lightning which perhaps could be of use:

• https://git.drupalcode.org/project/lightning_core/blob/8.x-3.x/tests/contexts/EntityContext.behat.inc
• https://git.drupalcode.org/project/lightning_media/blob/8.x-3.x/tests/contexts/MediaEntityContext.behat.inc

[pfrenssen]

Yeah they should not do automatic cleanup either, but clean them up manually at the end. When doing a UI test then entities created through the UI should be deleted through the UI. Cleaning up things automatically will hide potential bugs regarding handling of orphaned references etc.