fix(codegen): thread specialized_from through #7627's new Class fixture

Rebasing #7575 onto v0.5.1357 merges CLEANLY as text but does not compile:
#7627 added a `perry_hir::Class` literal in codegen/emission_order_tests.rs,
and #7575 adds a field to that struct. Textual mergeability is not a merge
check when one side widens a struct the other side constructs.

Author: Ralph Küpper

crates/perry-codegen/src/codegen/emission_order_tests.rs

@@ -297,6 +297,7 @@ fn plain_class(id: u32, name: &str, method: Function) -> Class {
         aliases: Vec::new(),
         is_nested: false,
         alloc_width_hint: 0,
+        specialized_from: None,
     }
 }