1070585: GUI no longer locks on connection test. Adds cancel button.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1070585

_reset_socket_timeout, test_connection could be a separate class. Nothing is really shared with NetworkConfigDialog. It could even be a subclass of threading.Thread. If that would make testing
easier, I'd try that. If not, don't worry about it.