minidns supports MX records, but today it hard-codes priority=10.
This is limiting when creating test scenarios that depend on having
different MX priorities.
This patch adds support for specifying the priority in MX records.
This patch adds a new integration test to cover SPF checks. The main
goal is not to cover the SPF parsing, since that's handled by the
library already, but the higher level aspects: that the mails are indeed
rejected, that the DSN looks reasonable, etc.