selftests: drv-net: gro: use SO_TXTIME to schedule packets together

Longer packet sequence tests are quite flaky when the test is run
over a real network. Try to avoid at least the jitter on the sender
side by scheduling all the packets to be sent at once using SO_TXTIME.
Use hardcoded tx time of 5msec in the future. In my test increasing
this time past 2msec makes no difference so 5msec is plenty of margin.
Since we now expect more output buffering make sure to raise SNDBUF.

Note that this is an opportunistic reliability improvement which
will only work if the qdisc can schedule Tx time for us (fq).
Fiddling with qdisc config was deemed too complex, so it's not
part of the patch.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20260318033819.1469350-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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