support/testing: move BRTest initialisation to __init__

BRTest's setUp() method contains a few assignments that initialize its
member variables. Since we will want to use these in test case
overrides, move them to the __init__ function.

Also allow the config member to be overridden, rather than always
taking the class member.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>