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 , better to be reviewed there.
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.

[link2xt]

IDK about introducing different behavior for tests though

This is already discussed in #8008, we indeed don't want to have conditional compilation or hooks just for tests. Just checking that MDN is in smtp_mdns table would be enough.

[link2xt]

This is the actual fix and it is likely good as long as other code is not changed just for testing.

[iequidoo]

For the record, i've removed conditional compilation, but just in case if changes still look complex, i'm going to replace the changes to smtp and the exisitng Rust tests with a JSON-RPC Python test, it should fail because MDNs aren't appearing on IMAP instantly.