support/testing: unbreak run-tests -l

In commit b78b5046, the initialisation
of BRTest.builddir was moved to the __init__ function. However, it is
set based on BRTest.outputdir and that is only set when the -o argument
is given to run-tests. When called as "run-tests -l", there is no -o
argument so BRTest.outputdir remains unset.

To fix, keep BRTest.builddir at None when BRTest.outputdir is None.

While we're at it, drop the direct access to the class member. If a
subclass wishes to set outputdir to something else before calling
BRTest.__init__, they are free to do so.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reported-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>