testing/infra: Add BRConfigTest as superclass of BRTest

The git tests don't need to do a full build, they only need to do a
configure and download and/or legal-info. More tests of that type will
be added in the future. Therefore, we want to have a test base class
that doesn't automatically do a full build in the setUp().

Add this new class as a superclass of the existing BRTest class, so we
don't need to update existing tests. Only the code in run-tests that
iterates over all subclasses of BRTest has to be adapted to use
BRConfigTest instead.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@datacom.ind.br>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>