* Fix newlines in JSON output
This changes the JSON output to be more consistent about where newlines are included. Previously it only included them between lines in a multiline diff. That meant single line changes were treated a bit weirdly. This changes it to append a newline to every line.
When feeding the results into `arc lint` this behaves correctly. I have only done limited testing though, in particular there's a possibility it might not work with files with `\r\n` endings (though that would have been the case before too).
Fixes#4259
* Update tests
# Conflicts:
# tests/writemode/target/output.json
* change layout so that test group `foo` has source files in
`test/foo/source` and target files in `test/foo/target`.
* use significant comments to specify write mode instead of threading
Option<WriteMode>
