tests: add test for post-fakeroot script

tests: add test for post-fakeroot script

Currently, only post-build and post-image scripts were tested, each with
their own test-script.

The two test-scripts only differ in the name of the log file they
create, and it is based on the name of the script, so it is easy to
share the script.

This allows us to easily re-use it for testing post-fakeroot scripts.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Reviewed-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Tested-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>