selftests: net: bridge: Parameterize ageing timeout

Allow the ageing timeout that is set on bridges to be customized from
forwarding.config. This allows the tests to be run on hardware which
does not support a 10s timeout (e.g. mv88e6xxx).

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>