testimage.bbclass: correctly process SIGTERM

Python's unittest will not propagate exceptions upside
of itself, but rather will just catch and print them.

The working way to make it stop is to send a SIGINT
(e.g. simulate a ctrl-c press), which will make it exit
with a KeyboardInterrupt exception.

This also makes pressing ctrl-c twice from bitbake work
again (previously hanging instances of bitbake and qemu were
left around, and bitbake would no longer start until they
were killed manually).

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>