Codegen: add @qltest.test_with

This allows to group together related AST classes to reuse the same
test source and extraction. For example this is useful for
`EnumDecl/EnumCaseDecl/EnumElementDecl`, where this is applied to.

swift/schema.py

@@ -107,6 +107,7 @@ class Stmt(AstNode):
 class GenericContext(Element):
     generic_type_params: list["GenericTypeParamDecl"] | child
 
+@qltest.test_with("EnumDecl")
 class EnumCaseDecl(Decl):
     elements: list["EnumElementDecl"]
 
@@ -247,6 +248,7 @@ class Callable(Element):
 class Function(GenericContext, ValueDecl, Callable):
     pass
 
+@qltest.test_with("EnumDecl")
 class EnumElementDecl(ValueDecl):
     name: string
     params: list[ParamDecl] | child