Followup from conversation within #1920666: Allow additional external libraries with composer.

This feature would add the ability for testbot to execute a 'composer install' during the installation process for D8 core & contrib modules.

This would likely require a few modifications:
i) formalize the inclusion of drush in testbot (currently, it's really only there as a diagnostic/troubleshooting tool, and could use some updating)
ii) Install/add composer to the testbot build
iii) modifications to the install() method inside review/drupal/

Of course, a next-generation testbot platform which natively supports the running of pre-execute scripts during the build process, where this becomes a test property change instead of an underlying code change, would be preferable. :)