pkg/subsystem: rewrite the inference logic

Let's just accept that we cannot fully trust guilty paths and try to
increase the weight of subsystems extracted from reproducers.

Instead of taking all subsystems that have received the highest number
of votes, take all which have received >= 33%. This will reduce noise
and in almost all cases limit the number of assigned subsystems to 2.

If there are >= 3 reproducers that point to exactly the same set of
subsystems, give them a preference. But still take one subsystem from
guilty paths if there's one that's mentioned >= 66% times.

The numbers themselves are somewhat arbitrary, but hopefully this will
improve the quality of subsystem inference.

Add some more tests.

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