add rpmdevtools as build dep for el5

9feb9cf4 added rpmdevtools as an install depedency;
it is also needed as a build dep for current functional tests.
DRY up test harness to use spec file for build deps

Ensure build deps are consistent across the spec file and test harness.

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 changes the scope of that assumption
to keep docker images small.