sanity-check: split out into a separate shell script

This makes it easier to run outside LAVA.

No functional changes intended, even where the checks I'm copying
are wrong (which will be fixed in subsequent commits).

Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D3647