Re-write and update tests for HMAC DRBG

[mborland]

Closes: #227

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.94%. Comparing base (4bfcc53) to head (7a5fd64).
Report is 18 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop     #237    +/-   ##
=========================================
  Coverage    99.94%   99.94%            
=========================================
  Files           87       89     +2     
  Lines         3389     3585   +196     
=========================================
+ Hits          3387     3583   +196     
  Misses           2        2            

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