remove intermediate containers in test harness

Save disk space on workstation.
DRY up test harness to use spec file for build deps

This makes it easier to assure all build deps are correct in both
spec and Dockerfile. Do not guess.

Note: http://fedoraproject.org/wiki/HOWTOFindMissingBuildRequires
says that some packages should not be BuildRequires because
"they would occur too often" and are "considered the minimum build
environment." However, Docker sort of changes the scope of that
belief since we strive to keep docker images small.