Deflakify PMTUTest.

This test would fail when it picked the same IP address more than
once during the test. Fix this by keeping track of all the IP
addresses used during the test and never reusing them.

While I'm at it, also add more entropy to our random IPv4
destinations.

Test: ./multinetwork_test.py PMTUTest passed 100 times in a row on net-next.

Bug: 31119353
Change-Id: Id49b5601d839f2427a6068137460fd2d7ceb1f03