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