GM_STEPS=20, 1-12 or 6,20 takes a subset of the suite, which is seconds
instead of two minutes while one step is being written. Steps live off
what the earlier ones leave behind, so a subset that skips a prerequisite
fails on purpose. The filter had been written as a throwaway script three
times; it belongs in the runner.
The README stops calling a remote browser the way this suite is meant to
be run: with the GPU in use a local run costs ~124 s and about 0.8 of the
four cores here, so GM_REMOTE is a convenience, not a requirement.