Avoid using false(1) in regression tests

Avoid using false(1) in regression tests

The POSIX standard only requires false(1) to return a "non-zero" exit
code; almost all systems return 1 but some (Solaris!!!) return 255 or
possibly even other values.  Use our helper "fail" instead.

* tests/thelp.pl: Have the "fail" command obey -q.
* tests/scripts/features/parallelism: Helper -q no longer prints fail.
* tests/scripts/targets/POSIX: Replace false with #HELPER# -q fail 1.
* tests/scripts/variables/MAKEFLAGS: Ditto.
* tests/scripts/variables/SHELL: Ditto.