test: Refactor lengthy processAlert tests (#1335)

This PR refactors some common setup out of each `processAlert` integration test, as these tests are becoming very lengthy and hard to understand and write.

- Common mocks are now performed in beforeEach() instead of in each test
- `setupSavedSearchAlertTest` and `createAlertDetails` helpers are introduced to create the objects each test case depends on (connection, source, team, savedSearch, client, webhook, alert, AlertDetails)
- `processAlertAtTime` helper is introduced to run `processAlert` at a particular `now` time value, after querying the AlertHistory records that exist at that time.

Helps with HDX-2568

Author: pulpdrew