This is a red flag, since this should never be required.  It turns out I regressed IMAP like a year ago, making it so ensureEssentialFolders basically never gets called.  I filed https://bugzilla.mozilla.org/show_bug.cgi?id=937483 on this.

Probably the right trade-off between correctness and laziness is to:

- Have Pop3Account explicitly call _learnAbout folder for sent and trash like CompositeIncomingAccount does for the inbox.  (That code is really just a poor man's first-run code.  It could also be factored out into an explicit first-run method.)

- Leave the other ensureEssentialFolders and createFolder logic as it is because of the weird unit tests and that's an annoying thread to pull on right now.

Accordingly, I do think we want to remove this helper method from here and the calls to it from test_mutation.js.  The only testing case that should then depend on ensureEssentialFolders working would be real IMAP server tests against an account where someone did a bare-bones dovecot setup.  In that case we would expect the account setup process with its expected syncFolderList call to just do that.