Almost backout afcd46716d6a which breaks _select_best raising an ambiguity exception in debug mode.
The problem is, before afcd4, *tests* ran in debug mode and we want
this (e.g. we want to show, rather than swallow, select ambigüities).
We juste replace the bogus __init__(vreg.config) by __init__(True), which is
practically equivalent and also much more clear.