selftests: drv-net: gro: break out all individual test cases

selftests: drv-net: gro: break out all individual test cases

GRO test groups the cases into categories, e.g. "tcp" case
checks coalescing in presence of:
 - packets with bad csum,
 - sequence number mismatch,
 - timestamp option value mismatch,
 - different TCP options.

Since we now have TAP support grouping the cases like that
lowers our reporting granularity. This matters even more for
NICs performing HW GRO and LRO since it appears that most
implementation have _some_ bugs. Flagging the whole group
of tests as failed prevents us from catching regressions
in the things that work today.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260113000740.255360-7-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>