Remove pretty_assertions dependency

It was only used in two places. Rather than adding `use
pretty_assertions::*` everywhere, it's easier to remove, and it
removes two additional dependency crates.

src/mimefactory.rs

@@ -1412,7 +1412,6 @@ mod tests {
     use crate::test_utils::{get_chat_msg, TestContext};
 
     use async_std::fs::File;
-    use pretty_assertions::assert_eq;
 
     #[test]
     fn test_render_email_address() {