fix: Tombstone MDN before sending it (#8252)

[iequidoo]

Otherwise, when it appears on IMAP, it will mark chat messages as seen/noticed even if
markfresh_chat() is called meanwhile.

test: Add function to queue MDN into smtp is extracted from #8008
Fix #8252

[WofWca]

Sounds like this should work but I'm probably not the right person to ask.

IDK about introducing different behavior for tests though, it doesn't seem like something we often do at least. At least some comments about could be useful? But maybe it's obvious to someone who knows what they're reading.

[iequidoo]

IDK about introducing different behavior for tests though, it doesn't seem like something we often do at least.

The smtp code (send_mdn_rfc724_mid() particularly) isn't tested at all in Rust currently. It's better to test it somehow than not test it at all, and running an SMTP server in Rust tests isn't what we want.