Test: Pretty the failure output

Signed-off-by: Alexander Berntsen <alexander@plaimi.net>
Signed-off-by: Stian Ellingsen <stian@plaimi.net>
Reviewed-by: Stian Ellingsen <stian@plaimi.net>
Add Mk-prefix to data constructors

Type constructors and data constructors should not share the same
namespace. This was a horrible idea, and inevitably leads to confusion
in every sophisticated codebase. Let's avoid it from hereon.

Signed-off-by: Alexander Berntsen <alexander@plaimi.net>
Signed-off-by: Stian Ellingsen <stian@plaimi.net>
Reviewed-by: Stian Ellingsen <stian@plaimi.net>