selftests: net: bridge: increase IGMP/MLD exclude timeout membership interval

When running the bridge IGMP/MLD selftests on debug kernels we can get
spurious errors when setting up the IGMP/MLD exclude timeout tests
because the membership interval is just 3 seconds and the setup has 2
seconds of sleep plus various validations, the one second that is left
is not enough. Increase the membership interval from 3 to 5 seconds to
make room for the setup validation and 2 seconds of sleep.

Fixes: 34d7ecb3

 ("selftests: net: bridge: update IGMP/MLD membership interval value")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>