Adjust test build process to allow testing with JS libs

The widget that uses the Spectrum external JS Library has known problems. Automatically testing it requires having the library available. However, can't require it because it is not available via standard packagist or Drupal packagist.

Solution:
So we need to add asset packagist, configure composer to use that with oomphinc/composer-installers-extender and then add the required external libraries. This should be doable with the container_command array within a drupalci.yml build file.