selftests/net: convert cmsg tests to make them run in unique namespace

Here is the test result after conversion.

]# ./cmsg_ipv6.sh
OK
]# ./cmsg_so_mark.sh
OK
]# ./cmsg_time.sh
OK

Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>