Workaround while the scheduler doesn't handle priority tasks

Summary: This should make tests more repeatable across different environments.

Test Plan: Run tests with a non-empty .gitconfig, for instance having commit signing turned on

Reviewers: ardumont

Differential Revision: https://forge.softwareheritage.org/D310

Summary:
When running the tests, we expect the local MTA to be working. That's not something that we can assume.

The notification tests all mock the SMTP object to check whether it's called
properly, so we don't really need to connect on backend initialization.

Test Plan:
Run tests with local MTA disabled, make sure they still pass.

Group Reviewers: Reviewers

Reviewers: ardumont

Reviewed By: ardumont

Differential Revision: https://forge.softwareheritage.org/D308

Closes T937

Moved here:
https://wiki.softwareheritage.org/index.php?title=Vault_Blueprint