selftests: forwarding: bridge_mdb: Make test more robust

Some test cases check that the group timer is (or isn't) 0. Instead of
grepping for "0.00" grep for " 0.00" as the former can also match
"260.00" which is the default group membership interval.

Fixes: b6d00da0

 ("selftests: forwarding: Add bridge MDB test")
Reported-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Closes: https://lore.kernel.org/netdev/adc5e40d-d040-a65e-eb26-edf47dac5b02@alu.unizg.hr/
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/r/20230808141503.4060661-18-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>