[TRIVIAL] Add unit test for redundant payment

[Bronek]

High Level Overview of Change

The preflight check to enforce that payee and payer must not be the same account have been in rippled at least 9 years, however we do not have unit test for this case. Add it.

Context of Change

Apparently other crypto ledgers are attacked by creating large number of transactions with the same payee and payer. We are immune to this type of attack, but it is not obvious. Let's make it clear and also improve unit test coverage.

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)
• 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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fad9d63) 61.48% compared to head (bec1e8f) 61.49%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4860   +/-   ##
========================================
  Coverage    61.48%   61.49%           
========================================
  Files          797      797           
  Lines        70121    70122    +1     
  Branches     36238    36238           
========================================
+ Hits         43114    43119    +5     
- Misses       19763    19764    +1     
+ Partials      7244     7239    -5     

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[gregtatcam]

👍 LGTM

[Bronek]

@intelliot this is ready to merge