t: split 01-critic.t into multiple smaller files

This allows the critic tests to naturally be run in parallel as well
as running only a "single" of them.

Signed-off-by: Niels Thykier <niels@thykier.net>
t/runtests: Allow $singletest to be a dir for suite:scripts

This allows people to use (e.g.) onlyrun=01-critic and have runtests
run all critic tests.

Signed-off-by: Niels Thykier <niels@thykier.net>