selftests: A few improvements to fib_nexthops.sh

Add nodad when adding IPv6 addresses and remove the sleep.

A recent change to iproute2 moved the 'pref medium' to the prefix
(where it belongs). Change the expected route check to strip
'pref medium' to be compatible with old and new iproute2.

Add IPv4 runtime test with an IPv6 address as the gateway in
the default route.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>