Reorganize unit/integration tests and deduplicate

[0xAlcibiades]

Reorganize unit tests to maximize compatibility with audit threat model to easily add needed additional cases. Additionally, remove duplicate coverage, complete refactor to separate file organization.

We can now more easily extend fuzz coverage, and add a sweeping integration test if we are inclined, though fuzzing and invariant testing may preclude.

[codecov-commenter]

Codecov Report

Merging #168 (52bd232) into audit-fixes (27dddb5) will increase coverage by 1.07%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           audit-fixes     #168      +/-   ##
===============================================
+ Coverage        90.05%   91.12%   +1.07%     
===============================================
  Files                2        2              
  Lines              372      372              
  Branches            56       56              
===============================================
+ Hits               335      339       +4     
+ Misses              29       27       -2     
+ Partials             8        6       -2     

Impacted Files	Coverage Δ
src/OptionSettlementEngine.sol	99.18% <0.00%> (+1.63%)	⬆️

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[0xAlcibiades]

The 2 logic branches not covered by the unit tests weren't covered by the whole test suite to begin with, would be great to track them down @neodaoist.

[0xAlcibiades]

The 2 logic branches not covered by the unit tests weren't covered by the whole test suite to begin with, would be great to track them down @neodaoist.

Might be unreachable code, actually.

[Flip-Liquid]

@0xAlcibiades want to back out the changes to the actor files? Those will conflict with the changes I'm making, and it looks like they're not substantive changes