selftests: drv-net: gro: test GRO stats

selftests: drv-net: gro: test GRO stats

Test accuracy of GRO stats. We want to cover two potentially tricky
cases:
 - single segment GRO
 - packets which were eligible but didn't get GRO'd

The first case is trivial, teach gro.c to send one packet, and check
GRO stats didn't move.

Second case requires gro.c to send a lot of flows expecting the NIC
to run out of GRO flow capacity.

To avoid system traffic noise we steer the packets to a dedicated
queue and operate on qstat.

Link: https://patch.msgid.link/20260318033819.1469350-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>