Test that if an RA sets the same prefix as DNS, nothing happens.

This is not particularly likely to happen unless the pref64 RA is
sent by a different router than the main RA. But more tests are
always good, and this additional coverage will be more useful
in an upcoming change.

Bug: 150648313
Test: test-only change
Change-Id: I3316d49d42100800740afadc4edf0a13a4d8377c