Adding _registerHook / _runHooks functionality allow adding work start/end of test helpers

[scalvert]

Description

Adds new hooks API, allowing external code to be 'hooked' into test helpers. This adds extensibility points that will allow consumers to define additional, custom behaviors to happen at specific points in the test invocation.

A good example of where this could be used is for accessibility testing - hooking into the visit:end to run audits without requiring audits be explicitly written into tests themselves.

Details

Adds the following public API:

function registerHook(helperName: string, hook: Function): HookUnregister

This allows registering a hook for a specific helper. Hooks can be registered to run at the start of the function, or the end.

Example:

Registering a fillIn hook to be run at the start of the visit invocation:

registerHook('fillIn:start', (target, test) => {
  // validate that target has a specific label
});

This also incorporates the previous log addition, which logs DOM elements, to use the new hooks API.

[fivetanley]

This would be a very handy thing to have for our accessibility testing also (we're hoping to contribute some back to ember-a11y-testing but still learning a lot). Is there a path to this become a stable exported api? (I noticed the words "internal" and also exports with underscores so I assume I shouldn't use them, but I would love to be wrong)

[scalvert]

@fivetanley this is actually precisely one of the use cases we're looking to support! The plan is definitely to have this be a publicly, supported API. We typically iterate on this type of API actively before we publish it for general consumption, as that helps validate our assumptions about the functionality.

Please let us know if there's anything you need from these APIs that we're missing.

[scalvert]

@fivetanley this is a late follow-up, but not sure you've seen https://github.com/ember-a11y/ember-a11y-testing/blob/master/addon-test-support/setup-global-a11y-hooks.ts#L20, which is where this is used in ember-a11y-testing with the new global hooks functionality.