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/jingle_ft.rs

@@ -304,7 +304,7 @@ mod tests {
     }
 
     #[test]
-    fn test_received() {
+    fn test_received_valid() {
         let elem: Element = "<received xmlns='urn:xmpp:jingle:apps:file-transfer:5' name='coucou' creator='initiator'/>".parse().unwrap();
         let received = Received::try_from(elem).unwrap();
         assert_eq!(received.name, ContentId(String::from("coucou")));
@@ -313,7 +313,11 @@ mod tests {
         let received2 = Received::try_from(elem2).unwrap();
         assert_eq!(received2.name, ContentId(String::from("coucou")));
         assert_eq!(received2.creator, Creator::Initiator);
+    }
 
+    #[test]
+    #[cfg_attr(feature = "disable-validation", should_panic = "Result::unwrap_err")]
+    fn test_received_unknown_child() {
         let elem: Element = "<received xmlns='urn:xmpp:jingle:apps:file-transfer:5' name='coucou' creator='initiator'><coucou/></received>".parse().unwrap();
         let error = Received::try_from(elem).unwrap_err();
         let message = match error {
@@ -321,7 +325,10 @@ mod tests {
             _ => panic!(),
         };
         assert_eq!(message, "Unknown child in Received element.");
+    }
 
+    #[test]
+    fn test_received_missing_name() {
         let elem: Element =
             "<received xmlns='urn:xmpp:jingle:apps:file-transfer:5' creator='initiator'/>"
                 .parse()
@@ -335,7 +342,10 @@ mod tests {
             message,
             "Required attribute field 'name' on Received element missing."
         );
+    }
 
+    #[test]
+    fn test_received_invalid_creator() {
         let elem: Element = "<received xmlns='urn:xmpp:jingle:apps:file-transfer:5' name='coucou' creator='coucou'/>".parse().unwrap();
         let error = Received::try_from(elem).unwrap_err();
         let message = match error {
@@ -361,7 +371,7 @@ mod tests {
     }
 
     #[test]
-    fn test_checksum() {
+    fn test_checksum_valid() {
         let elem: Element = "<checksum xmlns='urn:xmpp:jingle:apps:file-transfer:5' name='coucou' creator='initiator'><file><hash xmlns='urn:xmpp:hashes:2' algo='sha-1'>w0mcJylzCn+AfvuGdqkty2+KP48=</hash></file></checksum>".parse().unwrap();
         let hash = vec![
             195, 73, 156, 39, 41, 115, 10, 127, 128, 126, 251, 134, 118, 169, 45, 203, 111, 138,
@@ -388,15 +398,22 @@ mod tests {
                 hash: hash.clone()
             })
         );
+    }
 
-        let elem: Element = "<checksum xmlns='urn:xmpp:jingle:apps:file-transfer:5' name='coucou' creator='initiator'><coucou/></checksum>".parse().unwrap();
+    #[test]
+    #[cfg_attr(feature = "disable-validation", should_panic = "Result::unwrap_err")]
+    fn test_checksum_unknown_child() {
+        let elem: Element = "<checksum xmlns='urn:xmpp:jingle:apps:file-transfer:5' name='coucou' creator='initiator'><file><hash xmlns='urn:xmpp:hashes:2' algo='sha-1'>w0mcJylzCn+AfvuGdqkty2+KP48=</hash></file><coucou/></checksum>".parse().unwrap();
         let error = Checksum::try_from(elem).unwrap_err();
         let message = match error {
             FromElementError::Invalid(Error::Other(string)) => string,
             other => panic!("unexpected error: {:?}", other),
         };
         assert_eq!(message, "Unknown child in Checksum element.");
+    }
 
+    #[test]
+    fn test_checksum_missing_name() {
         let elem: Element = "<checksum xmlns='urn:xmpp:jingle:apps:file-transfer:5' creator='initiator'><file><hash xmlns='urn:xmpp:hashes:2' algo='sha-1'>w0mcJylzCn+AfvuGdqkty2+KP48=</hash></file></checksum>".parse().unwrap();
         let error = Checksum::try_from(elem).unwrap_err();
         let message = match error {
@@ -407,7 +424,10 @@ mod tests {
             message,
             "Required attribute field 'name' on Checksum element missing."
         );
+    }
 
+    #[test]
+    fn test_checksum_invalid_creator() {
         let elem: Element = "<checksum xmlns='urn:xmpp:jingle:apps:file-transfer:5' name='coucou' creator='coucou'><file><hash xmlns='urn:xmpp:hashes:2' algo='sha-1'>w0mcJylzCn+AfvuGdqkty2+KP48=</hash></file></checksum>".parse().unwrap();
         let error = Checksum::try_from(elem).unwrap_err();
         let message = match error {