tests: Set calendar event source

Ever since commit e9e30138bd6 (2011!), the calendar only initialized
the header at construction time. Set an event source to trigger an
update of the content area, which is what the test is mainly intended
for.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/38

tests/interactive/calendar.js

@@ -23,6 +23,7 @@ function test() {
              { expand: true,
                x_fill: false, x_align: St.Align.MIDDLE,
                y_fill: false, y_align: St.Align.START });
+    calendar.setEventSource(new Calendar.EmptyEventSource());
 
     UI.main(stage);
 }