Run coverage on a subset of tox targets

[zerolab]

As per https://drive.google.com/file/d/1Ks5_RrT_J64hOtGsRK3MMdovM_otvxDR/view?usp=sharing

it makes sense to limit coverage to a couple of full tox targets (e.g. py311-wagtail5.2-django-4.2-{sqlite,postgres}, as well we only run the sqlite tests for the same target. This should provide enough assurance and coverage, while limiting resource waste/carbon footprint