selftest: bond: add new topo bond_topo_2d1c.sh

Add a new testing topo bond_topo_2d1c.sh which is used more commonly.
Make bond_topo_3d1c.sh just source bond_topo_2d1c.sh and add the
extra link.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>