support/tests: use relative test_dir

support/tests: use relative test_dir

When we list the available tests, we use test_dir, which is set from
the path of the script. However, when we run the tests, we use the
hard-coded path.

Ditto to find the config file.

For consistency, always use test_dir.

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>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>