multiline strings: respect whitespace

[lue-bird]

The generated test uses extra spaces after each newline. For

--> """type List a
-->     = Nil
-->     | Cons a (List a)"""

the generated test will look like this:

                (
                """type List a
                    = Nil
                    | Cons a (List a)"""
                )

when it should be

                (
                """type List a
    = Nil
    | Cons a (List a)"""
                )