ipv6: fix ipv6_routes_with_src

there was a bug, that src was not visible with ip -6 r command, and this test did not cover it

Also modified the test to cover the bug, where some bytes of destination route are zeroed.

This two bugs were related to wrong memory structure used for storing the IPv6 route settings.

https://bugzilla.redhat.com/show_bug.cgi?id=1727193

@Build:nm-1-18