check_whence.py: check the permissions

check_whence.py: check the permissions

A handful of in-tree scripts must have the execute bit, as well as all
directories. Everything else should not.

In the past we had multiple commits adding and removing execute bit(s),
so instead we can check before things get in-tree.

With all the firmware files updated to drop the bit (as of last commit),
we can add some tests to enforce it going forward.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>