tests: skip demandimport if disabled demandimport and setuptools and decorator (from ironpython) and pygments leads to lots of fail. If demandimport is disabled we should skip testing it...

hghave: add demandimport checking

hghave: make bzr checks stricter My bzr does not have bzrlib.revisionspec.RevisionSpec, and thus tests were failing because convert refused to believe in bzr, but hghave without this change thought it was available.