Add testing guideline

[kazuhitoyokoi]

I added testing guideline page which we discussed on the following wiki page.
If there is a suggestion about the contents, please add the comments.

• Wiki page
• Original document

[ryoichi-obara]

I support this PR because of users convenience.
I suppose to users want to focus main content, so better than referring both docs(page and wiki).
The head of this wiki page says "finish this discussion around March 10.", have you finished discussion?

[kazuhitoyokoi]

@ryoichi-obara Yes, the discussion has been finished. Currently, it is under review in this pull request.
@dceejay@knolleary Could you review this pull request when you have time?

[dceejay]

I must admit we don't currently do this for most nodes - This is sensible.
(If this is done correctly can we then add a line to run the test via npm in the package.json? or does that then require we have all the deps in every node (inc node-red...?))

[knolleary]

There is a general discussion over whether a module should include its test material when published to npm. In the wider node community it is a similar discussion to tabs vs spaces - ie, no clear winner and a lot of energy spent not coming to agreement.

[knolleary]

Istanbul is deprecated so we shouldn't included it here. (see https://www.npmjs.com/package/istanbul)

We have a general task to migrate the whole project over to something else, probably nyc.

[knolleary]

There is a general discussion over whether a module should include its test material when published to npm. In the wider node community it is a similar discussion to tabs vs spaces - ie, no clear winner and a lot of energy spent not coming to agreement.