Reorganize the Flatpak-related test cases

This reorganizes and tweaks the Flatpak test cases to better express their intent:

• Renames the flatdeb- prefix, which wasn't all that clear.
• Renames the legacy tests depending on mildenhall to match, and ensures they're tied to v2021.
• Rewords the contents of the tests to be more consistent and updated for the current state of things.

https://phabricator.apertis.org/T8170

**IMPORTANT NOTE: This should be merged in tandem with pkg/apertis-tests!264 (merged)

Signed-off-by: Ryan Gonzalez ryan.gonzalez@collabora.com