Update test module interface documentation

[mattias-p]

Purpose

I started out making this update because I felt we need a good and up to date definition of the test module interface.

Context

Having a clear definition of the test module interface will be useful when we design the custom test module feature and in turn when people make use of it.

Changes

The justification for this PR is the added and updated details. While this PR contains some amount of new details most of the diff comes from formatting and editing and moving things around. Some older language and terminology is also updated to be more precise and contemporary. I may have got a little carried away but I believe the end result is good.

How to test this PR

N/A

[mattias-p]

@matsduf @marc-vanderwal @tgreenx Please review. I've rebased this on top of latest develop.

[mattias-p]

@matsduf

Regarding "internationalization" and "localization", I didn't make this up. It's actually well established terminology in the industry. See https://en.wikipedia.org/wiki/Internationalization_and_localization.

Regarding the "Make few assumptions" section, I didn't make this up either. It is what our current documentation says. I simply moved it to a better place. It's good that you spotted these flaws in our documentation but I feel they're out of scope for this PR.

[matsduf]

Even if "internationalization" and "localization" are established terms the documentation could be less confusing if it concretely explains what it is all about.

[mattias-p]

@matsduf I believe the documentation is quite concrete already. It explains what translators and developers need to do and what they need to think about, and I believe we have a decent linking structure that directs people to the right places. I'll add a link to the Wikipedia page. Would that be enough or do you have anything else in mind?