DT-581: Added nightwatch support.

[danepowell]

Fixes #3673

Changes proposed

• Add a new command tests:drupal:nightwatch:run to run core Nightwatch tests.

Steps to replicate the issue

Set up a local environment for testing. I was using Lando.

• You'll need Yarn available globally.
• You'll either need to install Chrome 75 somehow, or use Yarn to upgrade Drupal core's version of Chromedriver to match your installed version of Chrome.

In blt.yml, set:

tests:
  drupal:
    test-runner: nightwatch

Run blt tests:drupal:run

[danepowell]

@greylabel would you like to review this, since you originally requested the feature?

[greylabel]

Thanks @danepowell