selftests: Drop 'pref medium' in route checks

The 'pref medium' attribute was moved in iproute2 to be near the prefix
which is where it applies versus after the last nexthop. The nexthop
tests were updated to drop the string from route checking, but it crept
in again with the compat tests.

Fixes: 4dddb5be ("selftests: net: add new testcases for nexthop API compat mode sysctl")
Signed-off-by: David Ahern <dsahern@gmail.com>
Cc: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>