Clean up Invoicing Tests

[nospame]

Technical Summary

Reorganizes and lightly refactors invoicing test suites. Specifically removes TestDomainInvoiceFactory because most of that functionality is already covered in TestInvoice, and in review here it became clear that the distinction was blurry and likely unnecessary.

Review by commit.

Safety Assurance

Safety story

Changes automated tests only, no change to functional code.

Automated test coverage

It's automated tests all the way down.

QA Plan

Nope.

Rollback instructions

• This PR can be reverted after deploy with no further considerations

Labels & Review

• Risk label is set correctly
• The set of people pinged as reviewers is appropriate for the level of risk of the change

[biyeun]

move to setUp instead of setUpClass, and likewise with tearDownClass

[biyeun]

(additionally, better to move tearDown functionality to cleanup classes, so that cleanup actually executes if an error occurs)

[nospame]

After working on this for a couple hours, it's looking pretty messy to try and move the subscription setup entirely out of the class method, based on various subclasses assuming (reasonably) that there is always a default subscription.

Rather than explicitly assigning subscriptions in each of those subclasses, I may end up instead, for test cases that need a unique subscription (rather than the default), explicitly replacing that default subscription instead of adding a new one (which I believe was @jingcheng16 's concern in test_community_invoice, that we were just generating a new subscription but leaving still leaving the old one intact)

[nospame]

Actually, that wouldn't quite work either, because then further test cases would be missing the expected class-subscription. It's specifically the TestBillingAutoPay classes that make heavy use of the class-subscription, so maybe I can restructure those a bit to work with a subscription that gets set per-test case.