selftests: forwarding: lib: Move here vlan_capture_{, un}install()

Move vlan_capture_install() and vlan_capture_uninstall() from
mirror_vlan.sh test to lib.sh so that it can be reused in other tests.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>