selftests: fib_tests: add more tests for metric update

This patch adds two more tests to ipv4_addr_metric_test() to
explicitly cover the scenarios fixed by the previous patch.

Suggested-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>