Refactor tests to be idempotent

[raphaellaude]

Description

• Currently the backend tests are not idempotent, so you have to keep the state of the test db in your head as you write tests. This is of course difficult and annoying, not to mention less clear / explicit since adding fixtures for the state needed by a test can actually break it, due to unique constraints.
• This PR runs all tests in an outer transaction, which is always rolled back (fixes #91).
• Finally, renames the persistent session to persistent_session so this is still an option (for e.g. larger gerrydb tables needed for testing).

Reviewers

• Primary: @bailliekova @nofurtherinformation
• Secondary: @mapmeld @mariogiampieri

Checklist

• [ ] Added/Updated related documentation (if applicable).
• [x] Added/Updated related unit tests (if applicable).

Screenshots (if applicable):

[nofurtherinformation]

This is looking nice and definite QOL improvement for testing.

I think just needs main merged (new unshatter test) and then any docs you see needed but otherwise lgtm!

[nofurtherinformation]

lgtm!