BF(PY3): do no compare against threshold p until checking if not None

otherwise with python3 leads to crash (while with python2 just an empty match).
So took this also as opportunity to remove unneeded rebinding to p_thr complicating
analysis.
BF(PY3): ugly workaround for testing presence of tests

did not understand where eval issue comes from, but that is anyways
not really needed for this simple test, thus just allow to return
just a list of tests without their instantiation
BF: guarantee that enable_ca is always processed before disable_ca

as detected with failures on python3 our assumptions on
order were not always fulfilled and we have no control
over them.  So this provides a really ad-hoc solution explicitly
sorting all kwargs
Merge pull request #220 from yarikoptic/bf/py3-fixes

Bf/py3 fixes: none of them is elegant and underlying problem stinks with ancient code