[cairo-test] Use CAIRO_OPERATOR_SOURCE when painting the group surface.

To correctly copy a surface onto the destination irrespective of its
content requires the SOURCE operator. Forgetting to do so here causes
uninitialized pixels to be mixed into the result and the failure of
many tests for the similar surface. Oops...

test/cairo-test.c

@@ -384,6 +384,7 @@ cairo_test_for_target (cairo_test_t			 *test,
 
     if (similar) {
 	cairo_pop_group_to_source (cr);
+	cairo_set_operator (cr, CAIRO_OPERATOR_SOURCE);
 	cairo_paint (cr);
     }