* Implement a new fixture `eddn_message_from_file()` for testing this.
Eventually this will get renamed to `eddn_message()`.
* Files containing one message each go under `tests/eddn_message`, with
sub-directories for `<schema>/<version>[/<event>]` and then the filename
is the nature of that file in that context.
Completely invalid files, i.e. not even valid JSON, can be placed in
the top level of `tests/eddn_message`.
* Use this for the `parse_and_error_handle()` "valid journal/1 scan" test.
* `eddn_message()` fixture, set up to return a method when used, such that
*that* can be called with a key to look up the approproiate test message.
* `test_messages` dictionary to support that.
