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This patch moves the test declaration into QC.hs, so that test.hs has to
be modified only when we add a new test category.

This is more of an exercise in QuickCheck than strong testing.

This patch adds a QuickCheck-based test infrastructure and initial tests
based on it. The PeerMap module has a 100% coverage ☺

Side-note: one has to read the source of QuickCheck to see how to use it
(especially the Batch submodule), the docs are not enough…