Drop the separate core test workflow

[h-mayorquin]

core-test.yml runs the core tests on the three OSes with Python 3.10, but all-tests.yml already does the same as its first test step (same events, same --group test-core install, plus Python 3.13) and stops the job before the expensive modules if core fails. This PR removes core-test.yml so every push to a PR runs three fewer jobs, and runs the core step of all-tests.yml through run_tests.sh so we keep the verbose output and the timing summary.
This is the last item of #3100 that I think is worth doing, so this should close #3100.