xmpp_parsers: --features disable-validation

It was broken in multiple ways:

- xso did not honour it: unknown children and attributes would cause a
  parse error even with `--features disable-validation` set on parsers.
  For this, we introduce a new feature flag on xso, `non-pedantic`,
  which defaults unknown children and attributes to discard instead of
  fail.

  Note that individual XSOs can still choose to be always pedantic or
  always lenient by explicitly declaring the intent via the
  `on_unknown_child` and `on_unknown_attribute` metas.

- Many tests in `xmpp_parsers` were broken with `--features
  disable-validation`. They now all pass while *still* being rn with
  `disable-validation` set: In that case, they test that parsing in fact
  succeeds.

parsers/src/message_correct.rs

@@ -62,6 +62,7 @@ mod tests {
     }
 
     #[test]
+    #[cfg_attr(feature = "disable-validation", should_panic = "Result::unwrap_err")]
     fn test_invalid_child() {
         let elem: Element =
             "<replace xmlns='urn:xmpp:message-correct:0' id='coucou'><coucou/></replace>"