Invite tests sporadically failing, claiming a re-use of the invite link

[hoverduck]

For the last few days we've had sporadic failures with the invite tests, claiming that the invitation link was already used. It needs investigation.

https://circleci.com/gh/Automattic/wp-e2e-tests/12872

[alisterscott]

I spent some time looking at this:

1. It's definitely reproducible locally with inviting followers, and most often happens with our e2eflowtesting3 account. Using another account like e2eflowtesting is very slow to accept, but doesn't throw the same error
2. I think it's more likely the accept API rather than a calypso thing
3. Accepting invites have always been slow - @ebinnion wasn't able to work out why at the time invites were implemented
4. We have some logic to try to log on again when accepting an invite - I think may make it worse and I'm trying removing this in Try not attempting to relogin for invite specs #709
5. If we can't work it out I reckon it would be best just to exclude the follower test for now

[alisterscott]

I've merged #709 which seems to improve things generally, and excludes the follower invite test for now.

[alisterscott]

This test is still failing: #751

Should we delete it or investigate further?

[hoverduck]

How likely does it seem that it's an issue an actual user will have? Do we need a Trac issue to drive some investigation on the API side?