Add a test for the unhappy path of signing

[dvdplm]

I noticed that the test for the signing protocol does not traverse the unhappy path, where signature verification fails and we generate a correctness proof for the verifying signer. So here's a test for that along some more testing utils.

[codecov]

Codecov Report

Attention: Patch coverage is 98.16514% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.51%. Comparing base (9f9a0c4) to head (9d83fdb).

Files with missing lines	Patch %	Lines
synedrion/src/cggmp21/protocols/signing.rs	98.16%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #150      +/-   ##
==========================================
+ Coverage   90.72%   92.51%   +1.78%     
==========================================
  Files          44       44              
  Lines        8066     8137      +71     
==========================================
+ Hits         7318     7528     +210     
+ Misses        748      609     -139     

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[dvdplm]

@fjarri A review on this would be nice, so we can get rid of CI failures due to missing coverage.