parsing: fix test for new xkbcommon version

The latest version of `xkbcommon` (0.8) fixes a bug where
`keysym_to_utf8` returned a string including the terminating NULL
character. The `unicode_keysym` test used to rely on this behaviour, so
we must update it so it still works with the new version.

Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/697>

src/data/parsing.rs

@@ -696,7 +696,7 @@ mod tests {
             xkb::KEYSYM_NO_FLAGS,
         );
         let keysym = xkb::keysym_to_utf8(keysym);
-        assert_eq!(keysym, "å\0");
+        assert_eq!(keysym, "å");
     }
     
     #[test]