selftests: net: py: extract tool logic

The main use of the ip() wrapper over cmd() is that it can parse JSON.
cmd("ip -j link show") will return stdout as a string, and test has
to call json.loads(). With ip("link show", json=True) the return value
will be already parsed.

More tools (ethtool, bpftool etc.) support the --json switch.
To avoid having to wrap all of them individually create a tool()
helper.

Switch from -j to --json (for ethtool).
While at it consume the netns attribute at the ip() level.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240429144426.743476-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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