Test the size of every struct defined here.

src/stanza_error.rs

@@ -289,6 +289,13 @@ impl From<StanzaError> for Element {
 mod tests {
     use super::*;
 
+    #[test]
+    fn test_size() {
+        assert_size!(ErrorType, 1);
+        assert_size!(DefinedCondition, 1);
+        assert_size!(StanzaError, 208);
+    }
+
     #[test]
     fn test_simple() {
         #[cfg(not(feature = "component"))]