Skip POINTOPOINT of IPv4 too instead of IPv6 only

	Fix following error on `utun*`:
	```
	  1) Error:
	TestSocket#test_udp_server:
	Errno::ECONNREFUSED: Connection refused - recvmsg(2)
	```

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67825 b2dd03c8-39d4-4d8f-98ff-823fe69b080e