selftests: drv-net: iou-zcrx: rework large chunks test to use common setup

selftests: drv-net: iou-zcrx: rework large chunks test to use common setup

Commit a32bb32d0193 ("selftests: iou-zcrx: test large chunk sizes")
and commit de7c600e2d5b ("selftests/net: parametrise iou-zcrx.py with
ksft_variants") landed at similar time. The large chunks test was
actually not included in the list of tests, so it never run.
We haven't noticed that it uses the old-style helpers
(_get_combined_channels, _get_current_settings, _set_flow_rule)
that were removed by the other commit.

Rework test_zcrx_large_chunks to reuse the single() setup function
and add it to the ksft_run cases list so it actually gets executed.

Link: https://patch.msgid.link/20260227171305.2848240-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>