selftest: A few cleanups for fib_nexthops.sh

Cleanups of the tests in fib_nexthops.sh
1. Several tests noted unexpected route output, but the
   discrepancy was not showing in the summary output and
   overlooked in the verbose output. Add a WARNING message
   to the summary output to make it clear a test is not showing
   expected output.

2. Several check_* calls are missing extra data like scope and metric
   causing mismatches when the nexthops or routes are correct - some of
   them are a side effect of the evolving iproute2 command. Update the
   data to the expected output.

3. Several check_routes are checking for the wrong nexthop data,
   most likely a copy-paste-update error.

4. A couple of tests were re-using a nexthop id that already existed.
   Fix those to use a new id.

Fixes: 6345266a ("selftests: Add test cases for nexthop objects")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>