tests/bsim/bt l2cap/stress: Increase runtime timeout

This test has been seen failing in the new runners
due to a (realtime) timeout.
Let's double the timeout so it does not.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>
(cherry picked from commit f4972347)
Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>