refactor: Rename and move const_arg_path

It wasn't testing the `const_arg` code path, it was actually hitting
const_param's default value code path, so move it to the right place
and rename it.

crates/parser/src/grammar/generic_params.rs

@@ -85,6 +85,9 @@ fn const_param(p: &mut Parser, m: Marker) {
 
         // test const_param_default_expression
         // struct A<const N: i32 = { 1 }>;
+
+        // test const_param_default_path
+        // struct A<const N: i32 = i32::MAX>;
         generic_args::const_arg_expr(p);
     }