Test for invalid email addresses in mass subscribe

[Terri]

Bhavesh Goyal provided a patch to improve the validation of email addresses in mass subscribe here: https://code.launchpad.net/~bhavesh-goyal093/postorius/fixed-invalid-email-acceptance/+merge/255447

This is a wishlist item to remind me that I'd like to have some Tox tests that attempt to put invalid addresses into the mass subscribe box and make sure that the appropriate error is returned.