tests: Improve BaseAppTests class for functional tests

Discussed in https://wiki.debian.org/FreedomBox/ProgressCalls/2021-10-09.

• Functional test classes should be subclasses.
• Should not be collected as a test, only the subclasses will be.
• Implement enable/disable test.
• Implement run diagnostics test.
  • Drop any diagnostics that are known to fail.
• Implement backup/restore test.
• Further work: Check the components of the app to determine tests.
• Further work: Allow pre-backup and post-backup hooks.