Refactor hello-world test with simplified structure and explanatory comments

[m-dango]

No description provided.

[trosel]

Coming from #297

Thanks for starting this @mienaikage !

Is it necessary to use the JSON system at all? Introducing pod and all that pomp doesn't seem necessary for such a trivial use case.

Also, I'm not even sure if this manner of testing would be considered a best practice in the Perl world. Should just be about 5 is checks, I think.

[m-dango]

The JSON is taken directly from Exercism's problem-specifications, which is automatically included whenever an exercise is generated on this track. It allows for exercises to be easily updated if a test case is added/removed/changed.

It may not be a necessity for this exercise in particular, but hello-world has undergone changes before, and canonical data from problem-specifications is used across almost every exercise in this track. Having it makes it easier to both maintain existing exercises and implement new ones.

Ideally $=data would be used instead, but that is not yet implemented on rakudo.

[m-dango]

I'm going to make a separate change to the generator so that the canonical data can be removed without the exercise having to be maintained separately from the rest of the exercises on the track.