Add more assertions to testStackedLinkProperties.

Check all routes that are added and removed instead of just some
of them. This is in preparation of an upcoming change that
switches to adding and creating routes by issuing direct calls to
netd.

Also rename the misleading ipv4Default route variable to
ipv4Subnet, which is what it actually is.

Bug: 142892223
Test: test-only change
Change-Id: I7d111382be215a926a7d7d4701bd3c3e94372b99