tls: add a test for a case that used to not work
tls: add a test for a case that used to not work

If you created a source on a GTlsConnection before handshaking, it
would stop working after the handshake finished.

https://bugzilla.gnome.org/show_bug.cgi?id=710691

GNOME/glib-networking,GNOME/glib-networking,GNOME/glib-networking