Add a system test for mixed-case data for the same owner

Add a system test for mixed-case data for the same owner

We were missing a test where a single owner name would have multiple
types with a different case.  The generated RRSIGs and NSEC records will
then have different case than the signed records and message parser have
to cope with that and treat everything as the same owner.

(cherry picked from commit 14e435b8140ce850aa03233b2144b8997d95eaf7)