fixes typo in LendingHelpers unit-test

[Tapanito]

High Level Overview of Change

Fixes a typo and does minor refactoring to overpayment unit tests in LendingHelpers_test.cpp.

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)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.3%. Comparing base (12c0d67) to head (08a949c).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #6215     +/-   ##
=========================================
- Coverage     79.3%   79.3%   -0.0%     
=========================================
  Files          839     839             
  Lines        71630   71630             
  Branches      8280    8279      -1     
=========================================
- Hits         56811   56795     -16     
- Misses       14819   14835     +16     

see 6 files with indirect coverage changes

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