Fix the icon-effect-missing reftest

This was comparing the included image-missing icon
with the one in the current icontheme on the test
system. Works fine as long as we don't change
the icons (which we just did). To avoid this, set
the icontheme to hicolor for this test, which does
not have the image-missing icon, so we end up getting
the builtin icon for both ui files.

testsuite/reftests/icon-effect-missing.ref.ui

@@ -2,6 +2,7 @@
 <interface>
   <object class="GtkWindow" id="window1">
     <property name="decorated">0</property>
+    <signal name="map" handler="reftest:set_icon_theme_hicolor" swapped="no"/>
     <child>
       <object class="GtkBox" id="box1">
         <child>

testsuite/reftests/icon-theme-hicolor.c

@@ -0,0 +1,35 @@
+/*
+ * Copyright (C) 2021 Red Hat, Inc
+ *
+ * Author:
+ *      Matthias Clasen <mclasen@redhat.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "config.h"
+
+#include <gtk/gtk.h>
+
+
+G_MODULE_EXPORT void
+set_icon_theme_hicolor (void)
+{
+  GtkSettings *settings;
+
+  g_test_message ("Attention: setting icon-theme to hicolor");
+
+  settings = gtk_settings_get_for_display (gdk_display_get_default ());
+  g_object_set (settings, "gtk-icon-theme-name", "hicolor", NULL);
+}

testsuite/reftests/meson.build

@@ -23,6 +23,7 @@ libreftest = library('reftest',
     'textview-border-windows.c',
     'textview-tags.c',
     'animation-direction.c',
+    'icon-theme-hicolor.c',
   ],
   link_with: libgtkreftestprivate,
   c_args: reftest_cflags,