selftests: drv-net: replace the nsim ring test with a drv-net one

selftests: drv-net: replace the nsim ring test with a drv-net one

We are trying to move away from netdevsim-only tests and towards
tests which can be run both against netdevsim and real drivers.

Replace the simple bash script we have for checking ethtool -g/-G
on netdevsim with a Python test tweaking those params as well
as channel count.

The new test is not exactly equivalent to the netdevsim one,
but real drivers don't often support random ring sizes,
let alone modifying max values via debugfs.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20251029164930.2923448-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>