selftests: net: cope with slow env in gro.sh test

The gro self-tests sends the packets to be aggregated with
multiple write operations.

When running is slow environment, it's hard to guarantee that
the GRO engine will wait for the last packet in an intended
train.

The above causes almost deterministic failures in our CI for
the 'large' test-case.

Address the issue explicitly ignoring failures for such case
in slow environments (KSFT_MACHINE_SLOW==true).

Fixes: 7d157501 ("selftests/net: GRO coalesce test")
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/97d3ba83f5a2bfeb36f6bc0fb76724eb3dafb608.1707729403.git.pabeni@redhat.com


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