selftests: forwarding: Suppress grep warnings

Suppress the following grep warnings:

[...]
INFO: # Port group entries configuration tests - (*, G)
TEST: Common port group entries configuration tests (IPv4 (*, G))   [ OK ]
TEST: Common port group entries configuration tests (IPv6 (*, G))   [ OK ]
grep: warning: stray \ before /
grep: warning: stray \ before /
grep: warning: stray \ before /
TEST: IPv4 (*, G) port group entries configuration tests            [ OK ]
grep: warning: stray \ before /
grep: warning: stray \ before /
grep: warning: stray \ before /
TEST: IPv6 (*, G) port group entries configuration tests            [ OK ]
[...]

They do not fail the test, but do clutter the output.

Fixes: b6d00da0

 ("selftests: forwarding: Add bridge MDB test")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/r/20240208155529.1199729-4-idosch@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>