test: Check headers absense straightforwardly

In the `test` cfg, introduce `MimeMessage::headers_removed` hash set and `header_exists()` function
returning whether the header exists in any part of the parsed message. `get_header()` shouldn't be
used in tests for checking absense of headers because it returns `None` for removed ("ignored")
headers.

src/mimefactory/mimefactory_tests.rs

@@ -893,10 +893,7 @@ async fn test_dont_remove_self() -> Result<()> {
     let mime_message = MimeMessage::from_bytes(alice, sent.payload.as_bytes(), None)
         .await
         .unwrap();
-    assert_eq!(
-        mime_message.get_header(HeaderDef::ChatGroupPastMembers),
-        None
-    );
+    assert!(!mime_message.header_exists(HeaderDef::ChatGroupPastMembers));
     assert_eq!(
         mime_message.chat_group_member_timestamps().unwrap().len(),
         1 // There is a timestamp for Bob, not for Alice