selftests: net: lib: Add a couple autodefer helpers

Alongside the helper ip_link_set_up(), one to set the link down will be
useful as well. Add a helper to determine the link state as well,
ip_link_is_up(), and use it to short-circuit any changes if the state is
already the desired one.

Furthermore, add a helper bridge_vlan_add().

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/856d9e01725fdba21b7f6716358f645b19131af2.1734540770.git.petrm@nvidia.com


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