[v9_18] Various gcovr clean-ups
Validation job: https://gitlab.isc.org/isc-projects/bind9/-/jobs/2930536
Backport of MR !7104 (merged)
Edited by Michal Nowak
Validation job: https://gitlab.isc.org/isc-projects/bind9/-/jobs/2930536
Backport of MR !7104 (merged)