Add more tests for fundraising web hooks

[bmispelon]

It seems that the code in fundraising.views.WebhookHandler doesn't have any tests associated with it.

It would be nice to have a basic suite of tests, at least for the "happy path". This could prevent bugs like the one fixed recently in e995213.

EDIT: Not sure how I managed to miss it, but we do have some tests: https://github.com/django/djangoproject.com/blob/main/fundraising/tests/test_views.py#L183
So I'm narrowing down the scope of this to adding tests for the hooks that are not currently being tested (like checkout_session_completed)