feat: mimeparser: Omit Legacy Display Elements (#7130)

Omit Legacy Display Elements from "text/plain" and "text/html" (implement 4.5.3.{2,3} of
https://www.rfc-editor.org/rfc/rfc9788 "Header Protection for Cryptographically Protected Email").

src/test_utils.rs

@@ -1679,7 +1679,6 @@ Until the false-positive is fixed:
     }
 }
 
-#[cfg(test)]
 mod tests {
     use super::*;