selftests: Clean forgotten resources as part of cleanup()

Several tests do not set some ports down as part of their cleanup(),
resulting in IPv6 link-local addresses and associated routes not being
deleted.

These leaks were found using a BPF tool that monitors ASIC resources.

Solve this by setting the ports down at the end of the tests.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>