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_args.rs

@@ -99,9 +99,8 @@ pub(super) fn const_arg_expr(p: &mut Parser) {
             expressions::literal(p);
             lm.complete(p, PREFIX_EXPR);
         }
-        // test const_arg_path
-        // type T = S<u32::MAX>;
         _ => {
+            // This shouldn't be hit by `const_arg`
             let lm = p.start();
             paths::use_path(p);
             lm.complete(p, PATH_EXPR);

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);
     }