ch-test: fails silently if parent of packed or unpacked images directory doesn't exist

If you set your --img-dir to a path that doesn't exist ch-test will terminate (set -e) without any information.

ch-test is supposed to make the image directory if it doesn't exist, however, since we source common.bash first, the following line common.bash triggers an exit 1.

+++ readlink -ef /var/tmp/jogas/0.15/img
++ ch_imgdir=