The `nomad fmt -check` command incorrectly writes to file because we didn't
return before writing the file on a diff. Fix this bug and update the command
internals to differentiate between the write-to-file and write-to-stdout code
paths, which are activated by different combinations of options and flags.

The docstring for the `-list` and `-write` flags is also unclear and can be
easily misread to be the opposite of the actual behavior. Clarify this and fix
up the docs to match.

This changeset also refactors the tests quite a bit so as to make the test
outputs clear when something is incorrect.