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)
• Refactor (non-breaking change that only restructures code)
• Performance (increase or change in throughput and/or latency)
• 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.1%. Comparing base (c4308b2) to head (6319dec).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #5383     +/-   ##
=========================================
- Coverage     78.1%   78.1%   -0.0%     
=========================================
  Files          795     794      -1     
  Lines        68598   68498    -100     
  Branches      8278    8277      -1     
=========================================
- Hits         53593   53495     -98     
+ Misses       15005   15003      -2     

see 7 files with indirect coverage changes

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[thejohnfreeman]

Tests build and pass on my machine. 👍

[thejohnfreeman]

This line is no longer needed if you're using src/tests instead of tests. I still recommend tests as a matter of convention.