Switching some unit tests to doctest

[vvysokikh1]

High Level Overview of Change

This PR is continuation of work done by @thejohnfreeman. We intend to move from our unit tests framework to doctest.

Context of Change

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Refactor (non-breaking change that only restructures code)
• [ ] Performance (increase or change in throughput and/or latency)
• [x] Tests (you added tests for code that already exists, or your new feature included in this PR)
• [ ] Documentation update
• [ ] Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• [ ] Release

API Impact

• [ ] Public API: New feature (new methods and/or new fields)
• [ ] Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• [ ] libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• [ ] Peer protocol change (must be backward compatible or bump the peer protocol version)

[thejohnfreeman]

Happy to help with this as I can. Will take a look this evening. Please communicate with me here if there's something you'd like to ask.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.1%. Comparing base (df6daf0) to head (f6fb86d). Report is 1 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #5383     +/-   ##
=========================================
- Coverage     79.1%   79.1%   -0.0%     
=========================================
  Files          817     816      -1     
  Lines        71705   71605    -100     
  Branches      8240    8240             
=========================================
- Hits         56716   56618     -98     
+ Misses       14989   14987      -2     

see 6 files with indirect coverage changes

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[bthomee]

@vvysokikh1 can you please address the review comments & resolve the conflicts? This change would be perfect for the next release.

[vvysokikh1]

@vvysokikh1 can you please address the review comments & resolve the conflicts? This change would be perfect for the next release.

Comments addressed