Refactor CookieCrumble to abstract test frameworks

[stuartjbrown]

Refactor the test framework specific logic / code into their own assemblies in order to avoid polluting the CookieCrumble project with test framework specific references. This means that a small amount of (global) setup is needed before using Snapshot.

• Added an interface ITestFramework to abstract the test framework specific logic
• Added 2 projects with the test framework specific implementation (CookieCrumble.MSTest and CookieCrumble.Xunit)
• Added a test project for MSTest and updated the existing one which uses Xunit

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.32%. Comparing base (c477e13) to head (b6d6814).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6537   +/-   ##
=======================================
  Coverage   74.32%   74.32%           
=======================================
  Files        2860     2867    +7     
  Lines      149842   149872   +30     
  Branches    17411    17414    +3     
=======================================
+ Hits       111373   111396   +23     
- Misses      32671    32673    +2     
- Partials     5798     5803    +5     

Flag	Coverage Δ
unittests	74.32% <ø> (+<0.01%)	⬆️

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