More tests: Threshold Sign

[afck]

The current ThresholdSign tests have been adapted from the previous Coin tests, i.e. they are mainly concerned with the pseudorandom distribution of the signatures.

We should add proper tests for threshold signing in general, with different message reordering strategies (random, lowest ID first, …) and with adversaries that send invalid messages (no messages, signatures with the wrong key, signatures with the wrong payload, …).