insane: Add a naive heuristic to detect test subdirectories

insane: Add a naive heuristic to detect test subdirectories

if there is a "test" or "tests" subdirectory at toplevel, this usually
means we have some unit tests available.

This test is very good at detecting handcrafted tests and I was not able
to find any false positive.

False positive can be dealt with the usual INSANE_SKIP mechanism

Signed-off-by: Jérémy Rosen <jeremy.rosen@smile.fr>
Reviewed-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>