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

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

I'd split out the text getting and splitting into a separate method, and test the parsing directly
(ie, without having to setup the rest of the widgets and calling write_values()). Then you can also
mock it in other tests to get host:port to specific values easily.