Normalize multi-line string dedenting

Most of the tests are written using `"\n".join()` syntax to write multi-line strings. cleandoc is also used in some, newer cases.

We should find the cleanest possible way to define such multi-line code snippet strings, and normalize. If we can use `dedent`, we should. If `cleandoc` is a better choice, then we should create an alias of it (`dedent_code`?) and use that.