selftests: drv-net: adjust tests before defaulting to shell=False

Clean up tests which expect shell=True without explicitly passing
that param to cmd(). There seems to be only one such case, and
in fact it's better converted to a direct write.

Reviewed-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250830184317.696121-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>