[PHPUnit 10] Provide a static alternative to @dataproviders using PHPUnit mocks in MailHandlerTest

This one was quite fiddly but got there in the end.

The change to ::getAuthenticatedMockMessage() is strange and I can't quite explain it. The mocked ContactFormInterface now fails its expectations if you leave it in place, but when I traced the code through ::getContactForm() was never called in the authenticated user case, so the expectations were never met in the first place, unsure how this was passing before.

...and managed to refactor it further from here to make the dataprovider much easier to follow.

This one was a little more but updates to use booleans and arrays vs MessageInterface does appear much cleaner. Will lean on the tests showing green that nothing broke.

LGTM

Committed and pushed 54da986560 to 11.x and d404c50e5c to 10.3.x and 3781898992 to 10.2.x. Thanks!

Backported to 10.2.x to keep test coverage aligned.

longwave → closed merge request !7573

Automatically closed - issue fixed for 2 weeks with no activity.