tests: RADIUS server unreachable with error handling

tests: RADIUS server unreachable with error handling

radius_{auth,acct}_unreachable tested some parts of RADIUS client code
error handling. However, they did not test everything since the send()
calls for unreachable port on localhost did not return an error (that
error was reported on receive side). Extend this with similar test cases
using unreachable IP address to get send() error returns covered as
well.

Signed-off-by: Jouni Malinen <j@w1.fi>