gh-136438: Make sure `test_ast` pass with all optimization levels (#136596)

Explicitly pass an `optimizer` parameter to the calls of `ast.parse/compile`, because if it is not provided, the interpreter will use its internal state, which can be modified using the `-O` or `-OO` flags.

Co-authored-by: Kirill Podoprigora <kirill.bast9@mail.ru>