tests: move unexported macro doctest into unit test (#616)

Nightly has begun running doctests for unexported macros as of
https://github.com/rust-lang/rust/pull/96630, which caused a doctest for
test_unpack_octets_4 which was previously ignored to be run. This broke
the CI because macros that are not exported with `#[macro_export]`
cannot be used from external crates (and thus cannot be doctested). This
change ignores the doctest and copies the relevant code into a unit
test.

Co-authored-by: David Koloski <dkoloski@google.com>

src/frame/mod.rs

@@ -11,7 +11,8 @@ use std::fmt;
 ///
 /// # Examples
 ///
-/// ```rust
+/// ```ignore
+/// # // We ignore this doctest because the macro is not exported.
 /// let buf: [u8; 4] = [0, 0, 0, 1];
 /// assert_eq!(1u32, unpack_octets_4!(buf, 0, u32));
 /// ```
@@ -25,6 +26,15 @@ macro_rules! unpack_octets_4 {
     };
 }
 
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test_unpack_octets_4() {
+        let buf: [u8; 4] = [0, 0, 0, 1];
+        assert_eq!(1u32, unpack_octets_4!(buf, 0, u32));
+    }
+}
+
 mod data;
 mod go_away;
 mod head;