selftests: forwarding: mirror_lib: Move here do_test_span_vlan_dir_ips()

Move the function do_test_span_vlan_dir_ips() from mirror_vlan.sh test
to a library file mirror_lib.sh to allow reuse. Fill in other entry
points similar to other testing functions in mirror_lib.sh, they will be
useful in following patches.

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