BridgeDB still isn't checking DKIM verification results properly

From this commit message for this unittest:

Also, "dunno" certainly isn't a valid DKIM signature.

See the ticket description for legacy/trac#12086 (moved) for how

15:30:31 DEBUG  L495:server.lineReceived()  > X-DKIM-Authentication-Results: dunno

ended up in the headers on an incoming email in the first place.