selftests: Move address validation in nettest

IPv6 addresses can have a device name to declare a scope (e.g.,
fe80::5054:ff:fe12:3456%eth0). The next patch adds support to
switch network namespace before running client or server code
(or both), so move the address validation to the server and
client functions.

IPv4 multicast groups do not have the device scope in the address
specification, so they can be validated inline with option parsing.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>