[mr/1173] adjust tests for new pref-src attribute in DHCPv4 routes

Make adjustments for NetworkManager!1173 (closed).

With that change, NetworkManager will set the "src" attribute for routes received via DHCPv4. See there, why it does so.

This has little effect on the test, except that a few checks no longer match. In most cases, just extend the regex to accept both old and new variants. A few tests were cloned/forked, so that newer/older versions explicitly test against the presence/absence of the src attribute.