Fix GenerateProof returning an invalid proof on a short deadline

[fasmat]

Changed GenerateProof to always return a valid proof if not cancelled even if it takes longer than deadline.

This allows to just pass time.Now() as deadline to the function (for instance in tests) to return the smallest valid proof.

[codecov]

Codecov Report

Attention: Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.9%. Comparing base (ccfa5da) to head (06970d3).

Files	Patch %	Lines
prover/prover.go	96.6%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #497     +/-   ##
=========================================
+ Coverage     80.6%   80.9%   +0.2%     
=========================================
  Files           27      27             
  Lines         1743    1768     +25     
=========================================
+ Hits          1406    1431     +25     
  Misses         216     216             
  Partials       121     121             

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