mtd: Make MTD tests cancelable

I always go nuts when I start an MTD test on a slow device and have to
wait forever until it finishes. From the debug output I already know
what the issue is but I have to wait or reset the board hard. Resetting
is often not an option (remote access, you don't want lose the current
state, etc...).

The solution is easy, check for pending signals at key positions in the
code. Using that one can even stop a test by pressing CTRL-C as
insmod/modprobe have SIGINT pending.

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>