Enable a custom test suite to be "plugged in"

[EmmaJP]

Summary

There may be a need for certain projects to create a bespoke suite of tests to check accessibility is met is quite specific ways that exceed the MAG requirements. Enabling a custom suite of feature files and accompanying tests to be "plugged in" could facilitate this without affecting the core purpose of bbc-a11y.

Context & Motivation

This potential requirement is coming from some work with Code GEL. The concept being that certain design patterns could require specific things of the code to be accessible and it would be good for products to check that their version of a pattern meets this requirement. A product using Code GEL patterns as a basis for their code would be able to use both bbc-a11y and a custom Code GEL test suite to check accessible code does not regress or break.

It could also facilitate products create their own custom suite of tests for accessibility and running them alongside the bbc-a11y checks, thereby combining the results in one place.