INFRA-124 - Support for paratest

This PR introduces support for paratest. It includes a few key parts:

 * Add "civibuild clone-create" (etal) which allows one to create extra database instances that duplicate the build's original DB.
 * Add new dependencies, phpunit and paratest
 * Add new settings in "civitest.settings.d" which check if a unit-test is being executed within paratest. If so, setup a DB via"civibuild clone-create".
 * Add new settings in "civicrm.settings.d" and "CiviSeleniumSettings" which propagate the DSNs from the phpunit-tier to web-tier.