Add a test which 'builds' Debian's base-files.
Description
This MR adds another test to debian.benchmark which clones the Debian-stretch-bst project and builds the base-files/base-files.bst element.
I've been unable to figure out whether the local cache is reset after each test, if not, this will make our results inaccurate. I don't think we should land this MR until we're certain this is the case.