Provide conditional usage for 'setupMiddlewareReporter'

[drewlee]

Base implementation to address #266.

The basic idea is to output the environment variables in addon-test-support/cli-options.ts. If either one is specified, their corresponding value is overridden at build-time to true, and thus accessible from the browser/headless-browser environment.

This also provides a new useMiddlewareReporter API, to conditionally invoke setupMiddlewareReporter.

import { useMiddlewareReporter, setupMiddlewareReporter } from 'ember-a11y-testing/test-support';

if (useMiddlewareReporter()) {
  setupMiddlewareReporter();
}

Either one of these invocations will make useMiddlewareReporter return true, otherwise it's false:

ember test --query "enableA11yMiddlewareReporter=true"
ENABLE_A11Y_MIDDLEWARE_REPORTER=true ember test

Either one of these invocations will force a11y audit:

ember test --query "enableA11yAudit=true"
ENABLE_A11Y_AUDIT=true ember test

Ran this implementation with several apps to verify builds and tests work as expected.

[scalvert]

Is this still a draft or do you think it's ready, @drewlee?

[drewlee]

If you don't see any glaring issues, then I can pull this out of draft @scalvert. These changes are pretty stable, so I will go ahead in the interest of collab.

[scalvert]

If we rename this to useMiddlewareReporter, should we also rename this const to USE_A11Y_MIDDLEWARE_REPORTER?

[drewlee]

I'd rather keep it as is, so it's consistent with ENABLE_A11Y_AUDIT and enableA11yAudit=true.

[scalvert]

Overall this looks good. Thanks, @drewlee!