Write Tests for Webhooks

[diego-escobedo]

Currently working on #639, which adds tests for invoice.paid and invoice.past_due. Necessary to write some more tests for the other ones we have so far using the same patching scheme.

[djanda97]

Hi @diego-escobedo, I would be interested in adding more webhook tests. After taking a look at backend/metering_billing/tests/test_subscription.py, I can't find the TestInvoiceWebhooks class that was added in #639. It appears to have been deleted/overwritten in #615. Is the webhook functionality covered by another test?

[diego-escobedo]

Hey David! Unfortunate side effect of such a big merge. Currently I don't believe there's any tests covering that functionality so we would have to restore those + add new ones