Fix #1474 check for URI scheme in DMARC parser, add tests for DMARC parser

[uwekamper]

This PR adds a check if a DMARC policy has a common error where the rua is missing the scheme of the target URL:

• wrong: [email protected]
• correct: rua=mailto:[email protected]

Also adds some unit tests for the DMARC parsing subroutines.

See bug #1474