[SV 63315] tests: Simplify TERM signaling

[SV 63315] tests: Simplify TERM signaling

Tests that try to kill the make process were not behaving as expected
on OpenBSD: the signal was sent from make to its children but the
sleep didn't die.  Something odd about the way the shell treats TERM.
To reduce platform dependencies add "term" to the helper tool and run
that instead of kill / sleep.

* tests/thelp.pl: Add a new operation "term" that takes a PID.
* tests/scripts/features/output-sync: Use it.
* tests/scripts/features/temp_stdin: Ditto.