Reorganize test suite

(help wanted: I'd love to have help with this, but if you're interested, please coordinate with me about it, since it will affect a lot of parts of codebase)

Goals:

- portable (prefer haskell over shell scripts)
- easy to add new tests
- easy to find all tests that relate to a particular syntax
- fast (both quick to run, and quick to recompile while developing)
- easy to see what the failure is when a test fails

New tests should:

- allow input Elm code specified as text
- allow expected output (formatted) Elm code specified as text
- (future) allow expected AST to be specified as JSON text
- be written in Haskell (not depend on `bash` scripts)

New tests **probably** should:

- allow examples to be specified for individual AST node types (instead of having all tests specify fully-valid Elm source files)
- allow example input/output to be specified in text files instead of being inline in haskell files
- (future) somehow be associated to the syntax/format documentation

**maybe** should:

- organize formatting code for each AST type next to the corresponding parser

Lessons learned:

- testing parser by having tests that verify haskell AST output is too hard to maintain