fail instead of segfault on unreadable config files

The report mentions "apt list --upgradable", but there are others which
have inconsistent behavior ranging from segfaulting to doing something
with the partial (and hence incomplete) data. We had a recent report
about sources.list (#818628), this one mentions prefences, the obvious
next step is conf files… so the testcase is adapted to check for all
three in file and directory versions and run a bunch of commands each
time which should all have more or less the same behavior in such a case
(aka error out).

Closes: 824503

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