Test for Upstream PR #3476

[MaggieFero]

Is your feature request related to a problem? Please describe.
Upstream PR bookwyrm-social#3476 (bookwyrm-social#3476) added a check blocking email domains on the blocklist from sending invite requests, and upstream determined no additional tests were needed for this.

Describe the solution you'd like
Because the problem solved by PR bookwyrm-social#3476 has been a particular difficulty for our server, we should add a test to ensure that this doesn't break if possible.

Describe alternatives you've considered
It might be that invite request flows are not currently supported by the test framework, in which case our options are to extend the testing framework to support invite request flows or to leave this untested and just expect that we'll see quite quickly in prod if it ever breaks. Between those, we'll probably want to leave it untested if extending the test framework would take more than 20-30 minutes, as that's the worst single impact from the problem this functionality solves.

Additional context
Because the main Bookwyrm instance using the upstream repo is open invite, this functionality is likely not as high priority for them as it is for us, an invite-required server where nearly all moderator intervention required (as opposed to desired, like adding new domains) is rejecting the same domains many times.