Improve separate test suite sourcing instructions
    
    Instructions given for creating a source archive for test suites can be
    improved slightly. Existing method creates the archive from checked
    out Git branch. This leaves open the possibility that there are local
    changes in the working tree, which end up packaged. The proposed
    method avoids the possibility by never checking out anything and relying
    on 'git archive' that operates directly on committed items in the
    repository.