test: add test for default icon resource

wfassoc/tests/concept.rs

@@ -186,6 +186,9 @@ fn test_str_ref_str() {
 
 #[test]
 fn test_icon_rc() {
+    fn const_tester(icon: Result<IconRc, LoadIconRcError>) {
+        assert!(icon.is_ok())
+    }
     fn ok_tester(file: &str, index: u32) {
         let icon = IconRc::new(file, index, IconSizeKind::Small);
         assert!(icon.is_ok())
@@ -195,6 +198,9 @@ fn test_icon_rc() {
         assert!(icon.is_err())
     }
 
+    // Test 2 const value
+    const_tester(IconRc::GENERIC_APPLICATION(IconSizeKind::Small));
+    const_tester(IconRc::GENERIC_APPLICATION(IconSizeKind::Small));
     // We pick it from "jpegfile" ProgId
     ok_tester("imageres.dll", 72);
     ok_tester("notepad.exe", 0);