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.

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