test-userdata-dir-invalid-home: Unset XDG_DATA_HOME

On some test environments XDG_DATA_HOME may be configured with a path that
actually exists, so we unset it to actually get the temporary directory.

libgnucash/core-utils/test/test-userdata-dir-invalid-home.c

@@ -85,6 +85,7 @@ main(G_GNUC_UNUSED int argc, G_GNUC_UNUSED char **argv)
      * directory in that case. */
     g_setenv("HOME", homedir, TRUE);
     g_setenv("GNC_DATA_HOME", testdatahome, TRUE);
+    g_setenv("XDG_DATA_HOME", "", TRUE);
     g_free (homedir);
     g_free (testdatahome);