Tweak memo text size property test to pass

Previously, there was a case where we were trying to exercise text sizes
that were too large, and this caused the newly added constructor to
fail. This addess extra logic to deal with this.

crates/core/transaction/src/memo.rs

@@ -380,7 +380,12 @@ mod tests {
             let memo_address = Address::dummy(&mut rng);
             let memo_text = s;
             let memo = {
+                let text_len = memo_text.len();
                 let memo = MemoPlaintext::new(memo_address, memo_text);
+                if text_len > MAX_TEXT_LEN {
+                    assert!(memo.is_err());
+                    return Ok(());
+                }
                 assert!(memo.is_ok());
                 memo.unwrap()
             };