Refactors vault invariant tests to test all asset types.

[Tapanito]

This PR pulls out Vault Invariants into a separate class, splits them by transaction type and, where applicable, extends the logic to test vaults with multiple asset types.

High Level Overview of Change

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 79.4%. Comparing base (bb529d0) to head (cb1f891).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #6269     +/-   ##
=========================================
- Coverage     79.4%   79.4%   -0.0%     
=========================================
  Files          839     839             
  Lines        71625   71625             
  Branches      8241    8236      -5     
=========================================
- Hits         56861   56855      -6     
- Misses       14764   14770      +6     

see 4 files with indirect coverage changes

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