test/py: expose config and log as session scoped fixture

If a test uses a fixture which is expensive to setup, the fixture can
possibly created with session or module scope. As u_boot_console has
function scope, it can not be used in this case.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>