Tests: add coverage report

[sergiogiro]

Print a coverage report at the end of make check. Opens the door to check for a minimum coverage, or checking that the coverage doesn't decrease.

Why this? I started exploring it, seemed possible, then it was one rabbit hole after the other. Next time I'll think twice before thinking "I'll finish this just because I started it".

[SamirTalwar]

Leaving this one in the capable hands of @kprimerakis.