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 92.72%. Comparing base (13eec60) to head (c0ba441).

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   90.89%   92.72%   +1.82%     
==========================================
  Files          45       45              
  Lines        8162     8233      +71     
==========================================
+ Hits         7419     7634     +215     
+ Misses        743      599     -144     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[dvdplm]

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

[fjarri]

This may have to be modified when #151 lands, but it's a good test for now.