Add a test for an invalid resource

This one uses unassigned codepoints in Unicode 3.2.

jid/src/lib.rs

@@ -880,6 +880,11 @@ mod tests {
         assert_eq!(full, equiv);
     }
 
+    #[test]
+    fn invalid_stringprep() {
+        FullJid::from_str("a@b/🎉").unwrap_err();
+    }
+
     #[test]
     fn jid_from_parts() {
         let node = NodePart::new("node").unwrap();