"""
What I assume has not been clear about my question - from the output I
see that there is, somewhere, a file named 'core' - but I can never find
it - and my assumption is that the test environment is cleaning up after
itself.
"""

Right, regrtest removes all temporary directories used for tests. You should try to extract the code run by the test and run it manually.

For example for test_recursion_limit(), you can use attached recurse.py.