Test setup for preserved whitespace on AST

[german1608]

From #1991 we noticed that we don't have tests to ensure that the preserved whitespace on some of our AST constructors is what we expect on other packages (dhall-docs, for instance).

To tackle this, we can have a golden test setup, similar to what we have in other packages (see dhall-docs and dhall-json error messages tests) that take the output generated from dhall haskell-syntax-tree --noted command and compare to the golden source files.

The test cases should handle the following constructors:

• Let bindings
• Lam bindings
• RecordField whitespaces
• (After merging feat(dhall-docs): add jump-to-definition support for record-like expressions #1991) FieldAccess whitespaces