tests: footprint: fix thread start races

The test granted access to the user work queue stack from the user
work thread; this was done by k_work_user_queue_start() so was
unnecessary.  Document why it's ok to grant other access after the
work thread was started.

Fix a race condition where the non-work user thread might have started
before it was given access to the resources it needs.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>