Merge branch 'selftests-drv-net-round-some-sharp-edges'

Jakub Kicinski says:

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selftests: drv-net: round some sharp edges

I had to explain how to run the driver tests twice already.
Improve the README so we can just point to it.
Improve the config validation.

v1: https://lore.kernel.org/r/20240424221444.4194069-1-kuba@kernel.org/
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Link: https://lore.kernel.org/r/20240425222341.309778-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>