test: fix domain with abort-on-uncaught on PPC

Add SIGTRAP and the corresponding exit code to the list of signals/exit
codes that are expected when running tests that throw an uncaught error
and have --abort-on-uncaught-exception enabled.

Also refactor a bit related comments so that they better reflect what's
actually happening.

Fixes #3239.

PR: #3354
PR-URL: https://github.com/nodejs/node/pull/3354


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>