support/testing: allow run-tests to be called from anywhere

support/testing: allow run-tests to be called from anywhere

Currently run-tests must be called from the Buildroot top directory.

Derive the top directory from the script path, so run-tests can be called from
any path.
As a consequence the test infra will always test the repo it belongs to.

Suggested-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>