selftests: drv-net: gro: add data burst test case

selftests: drv-net: gro: add data burst test case

Add a test trying to induce a GRO context timeout followed
by another sequence of packets for the same flow. The second
burst arrives 100ms after the first one so any implementation
(SW or HW) must time out waiting at that point. We expect both
bursts to be aggregated successfully but separately.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260402210000.1512696-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>