runtests: add options to set minimum number of tests, use them

runtests: add options to set minimum number of tests, use them

To detect mistakes made in the runtests framework that reduce
the number of test runs. Before this patch it could go undetected with
a green CI.

The minimum thresholds will need light maintenance going forward (either
bumping them periodically, or adjust if some may fell below minimums for
justified reasons). We may also make minimums tighter or looser, or more
job-specific.

Latest number of test runs for each job can be seen at Test Clutch:
https://testclutch.curl.se/static/reports/feature-matrix.html

Also:
- GHA: set minimums.

Assisted-by: Dan Fandrich
Follow-up to f2a75a14dd95e06cf9121c7d3bd5dc559748f350
Follow-up to bb1391f94394e635c1a5c58253e7a3d3b36bde57 #19510

Closes #19942