selftests: forwarding: Split mirror_gre_topo_lib.sh

Move generic parts of mirror_gre_topo_lib.sh into a new file
mirror_topo_lib.sh. Reuse the functions in GRE topo, adding the tunnel
devices as necessary.

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