Pull up following revision(s) (requested by ozaki-r in ticket #688):

tests/net/ndp/t_ndp.sh: revision 1.31
tests/net/if_tap/t_tap.sh: revision 1.8

Avoid setting IP addresses of the same subnet on different interface

If we do so, there will remain one route that is of a preceding address, but
that behavior is not documented and may be changed in the future.  Tests
shouldn't rely on such a unstable behavior.

(martin)