Additional unit tests for testing deletion of trust lines

[ckeshava]

High Level Overview of Change

This PR includes additional unit tests for testing deletion of trust lines.

Context of Change

The tests verify if the trust lines are removed from the owner directory once the settings are restored to defaults.

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

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)
  No change to APIs

[Bronek]

Nice tests, however they seem to focus on happy path only. Would be good to add several operations which are meant to fail, by design, and confirm they do fail. e.g. exceeding held amount or trying to set up trust line without authorization.

[ckeshava]

@Bronek I have added three new unit tests to validate the negative results in 2e0dfce

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.3%. Comparing base (58f3abe) to head (64005df).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #4886     +/-   ##
=========================================
- Coverage     71.3%   71.3%   -0.0%     
=========================================
  Files          796     796             
  Lines        66883   66883             
  Branches     10865   10868      +3     
=========================================
- Hits         47687   47683      -4     
- Misses       19196   19200      +4     

see 3 files with indirect coverage changes