tests: also trap SIGQUIT; other fixes

tests: also trap SIGQUIT; other fixes

Some shells seems to just ignore SIGQUIT under some circumstances,
even when it is not blocked; however, if the signal it trapped,
the trap gets correctly executed.

* tests/defs [$sh_errexit_works = yes]: Also trap SIGQUIT. Add
extensive explanation of why we do so.
* tests/self-check-cleanup.tap: Improve and fix to ensure that an
earlier failure does not prevent the rest of the test to execute.
Problem revealed by a failure in `self-check-cleanup.tap'.