tests: Add an assertion to guard against unexpected flags

This should not change the behaviour of the test, unless the test's
behaviour is changed in future. As spotted by cppcheck, which has
impeccable taste and a flair for the unexpected.

glib/tests/markup-parse.c

@@ -240,6 +240,8 @@ get_expected_filename (const gchar       *filename,
     expected = g_strconcat (f, ".expected", NULL);
   else if (flags == G_MARKUP_TREAT_CDATA_AS_TEXT)
     expected = g_strconcat (f, ".cdata-as-text", NULL);
+  else
+    g_assert_not_reached ();
 
   g_free (f);