lib/diag.rs refactor

This splits diag.rs, which was growing way too big for my taste, into a
number of submodules. This should help us compartmentalize tests better,
as well as use mod namespaces to shorten our struct/enum names.

lib/src/qmdl.rs

@@ -218,7 +218,7 @@ where
 mod test {
     use std::io::Cursor;
 
-    use crate::diag::{DataType, HdlcEncapsulatedMessage, test::get_test_message};
+    use crate::diag::{DataType, HdlcEncapsulatedMessage, diaglog::test::get_test_message};
 
     use super::*;