Don't hard-code special-use mailbox names for Yahoo accounts

Desipte Yahoo's IMAP CAPABILITIES not claiming to support SPECIAL-USE,
its LIST results actually include the appropriate flags, so just use
those.
7 jobs for wip/34-yahoo-fixes in 39 minutes and 40 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #202975
fedora

00:09:21

passed #202989
ubuntu

00:22:28

failed #202976
ubuntu

00:08:51

 
  Test
passed #202977
deb-package

00:07:50

canceled #202978
flatpak

 
  Deploy
skipped #202979
review
skipped #202980
allowed to fail manual
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