Make the Nat464Xlat tests more realistic.

464xlat will never be started on a network that is not connected,
or on a network that has no IPv6 address.

This is a no-op test-only change but it is necessary for an
upcoming change that violates some of the invalid assumptions
currently made by this test and causes it to fail.

Bug: 150648313
Test: test-only change
Change-Id: I41766e9adaa7c24454648b371e6e3cc647693be5