[test] update degenerate-pen test

pdiff was hiding a rgb24 failure here, as the test was drawing using
black ink on the default black background.  Instead, explicitly fill
the surface with white first.

test/degenerate-pen.c

@@ -60,6 +60,10 @@ cairo_test_t test = {
 static cairo_test_status_t
 draw (cairo_t *cr, int width, int height)
 {
+    cairo_set_source_rgb (cr, 1, 1, 1);
+    cairo_paint (cr);
+
+    cairo_set_source_rgb (cr, 0, 0, 0);
     cairo_set_line_join (cr, CAIRO_LINE_JOIN_ROUND);
 
     cairo_translate (cr, PAD, PAD);