(lrtable) Make the final_state field conditional on cfg(test)

This conditionally includes a field into `struct StateTable`.
Previously `clippy with --no-default-features -p lrtable` would produce
a warning about `final_state` never being read.  This was presumably
being suppressed by serde derive macros.  Since this field was only used
in the test cfg.

It seemed better to make it conditional rather than `#[allow(dead_code)]`.

lrtable/src/lib/statetable.rs

@@ -141,6 +141,7 @@ pub struct StateTable<StorageT> {
     prods_len: PIdx<StorageT>,
     tokens_len: TIdx<StorageT>,
     conflicts: Option<Conflicts<StorageT>>,
+    #[cfg(test)]
     final_state: StIdx<StorageT>,
 }
 
@@ -370,6 +371,7 @@ where
             prods_len: grm.prods_len(),
             tokens_len: grm.tokens_len(),
             conflicts,
+            #[cfg(test)]
             final_state: final_state.unwrap(),
         })
     }