Add various tests for current parsing

Add more varying testing for most parsing functions.
Tests emphasize whitespace, literals, timezones, and timezone
delimiters (colons and whitespace).

Add tests for multiple-byte characters and combining characters
in and around data and parsing formats.

These tests are added to aid humans verifying the next commit that
changes parsing behavior.

Issue #660

src/datetime/mod.rs

@@ -523,13 +523,18 @@ impl DateTime<FixedOffset> {
     /// RFC 2822 is the internet message standard that specifies the representation of times in HTTP
     /// and email headers.
     ///
+    /// The RFC 2822 standard allows arbitrary intermixed whitespace.
+    /// See [RFC 2822 Appendix A.5]
+    ///
     /// ```
     /// # use chrono::{DateTime, FixedOffset, TimeZone, NaiveDate};
     /// assert_eq!(
     ///     DateTime::<FixedOffset>::parse_from_rfc2822("Wed, 18 Feb 2015 23:16:09 GMT").unwrap(),
     ///     FixedOffset::east_opt(0).unwrap().with_ymd_and_hms(2015, 2, 18, 23, 16, 9).unwrap()
     /// );
     /// ```
+    ///
+    /// [RFC 2822 Appendix A.5]: https://www.rfc-editor.org/rfc/rfc2822#appendix-A.5
     pub fn parse_from_rfc2822(s: &str) -> ParseResult<DateTime<FixedOffset>> {
         const ITEMS: &[Item<'static>] = &[Item::Fixed(Fixed::RFC2822)];
         let mut parsed = Parsed::new();