doc: indicate that abort tests do not generate core files

The key thing about the tests in `test/abort` is that tests in that
directory do not generate core files (thanks to testcfg.py). Make a note
of that in the test README.

PR-URL: https://github.com/nodejs/node/pull/38422


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>