Test agreement with randomly generated keys

[justsmth]

Description of changes:

• Tests for "agreement" were largely using known test values or "KATs". This change adds a test using randomly generated keys.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.86%. Comparing base (c358484) to head (fc4f0c0).
Report is 38 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #464      +/-   ##
==========================================
- Coverage   95.80%   92.86%   -2.94%     
==========================================
  Files          61       62       +1     
  Lines        8143     8663     +520     
  Branches        0     8663    +8663     
==========================================
+ Hits         7801     8045     +244     
- Misses        342      362      +20     
- Partials        0      256     +256     

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