Lending Protocol: Fix Overpayment ValueChange calculation

[Tapanito]

Fixes overpayment calcuation, adds tests ad refactors to be without side effects.

[codecov]

Codecov Report

❌ Patch coverage is 93.65079% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.1%. Comparing base (5224636) to head (f9438cc).

Files with missing lines	Patch %	Lines
src/xrpld/app/misc/detail/LendingHelpers.cpp	96.4%	2 Missing ⚠️
src/xrpld/app/tx/detail/LoanSet.cpp	71.4%	2 Missing ⚠️

Additional details and impacted files

@@                       Coverage Diff                        @@
##           ximinez/lending-XLS-66-ongoing   #6114     +/-   ##
================================================================
- Coverage                            79.1%   79.1%   -0.0%     
================================================================
  Files                                 839     839             
  Lines                               71370   71367      -3     
  Branches                             8322    8324      +2     
================================================================
- Hits                                56453   56447      -6     
- Misses                              14917   14920      +3     

Files with missing lines	Coverage Δ
src/xrpld/app/misc/LendingHelpers.h	95.0% <ø> (ø)
src/xrpld/app/misc/detail/LendingHelpers.cpp	90.0% <96.4%> (-0.3%)	⬇️
src/xrpld/app/tx/detail/LoanSet.cpp	91.4% <71.4%> (+<0.1%)	⬆️

... and 1 file with indirect coverage changes

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