Add tests #28

Originally from github issue:

As poobrains matures, it's sensible to finally add some tests.

Flask has some testing stuff built-in - use it!

What can even be tested?

CRUD (directly via ORM, via web and via CLI)

This should be possible to do automagically for every Storable, i.e. to supply some automated tests for developers using poobrains

Form/CLI validation and coercion

ForeignKey cascade on update/delete (when #\2 is fixed)# ADD LINK TO GITEA ISSUE

Whether permissions work as intended

Whether Renderable.expose properly works

HTTP status codes

Do nonexistant pages actually throw a 404?

?

Issue from 2017 that was never updated. A good chunk of this should be implemented by the testing module, which at least tests installation, CRUD and permissions.