Added test tool to do UI snapshot testing

[ViMaSter]

DRAFT PR UNTIL BUILD WORKFLOW IS UPDATED TO UPLOAD SCREENSHOTS OF FAILED TESTS

Summary

To prevent issues like #312 from reoccurring I added a one-line tool that can be added to do snapshot testing similar to Jest.

When initially hit, Unity creates a screenshot, which needs to be committed to the repository. For all subsequent runs, Unity creates another screenshot and compares it with the one stored inside the repository.

Before/after screenshots and/or animated gif

Adding yield return ScreenshotDiff.TakeScreenshotOrCompare() to any test causes one these things to happen:

• if there's no "expected state" yet and the Editor is open...
  1. Unity takes a screenshot and stores it inside /Assets/Tests/PlayModeTests/Tools/Screenshots/{FullNameOfTest}/{TakeScreenshotOrCompareWasCalledInsideTest}.png
  2. Unity continues the test
  3. this file becomes the "expected state" of the UI at that point; commit this file to the repository
• if there's no "expected state" yet and the test is running on GitHub...
  1. Unity marks the test as failed as there's no "expected" state for Unity to compare against
     
• if the "expected state" exists...
  1. Unity takes a screenshot as "actual state" and stores it inside a temporary location
  2. Compares the "actual" with the "expected" state
     1. if it matches
        1. the test continues
     2. if it doesn't match
        1. Unity copies the expected and actual screenshot to /TestRunArtifacts/TestContext.CurrentTestExecutionContext.StartTime/{FullNameOfTest}.{TakeScreenshotOrCompareWasCalledInsideTest}
        2. Unity marks the test as failed and uploads both expected and actual screenshots
           
           

Testing instructions

not yet available

Additional information

• [ ] Changes UI
• [ ] Introduces new feature
• [ ] Removes existing feature
• [ ] Has associated resource:

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.34%. Comparing base (ea6a61f) to head (78ddf3d).
Report is 261 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #343      +/-   ##
===========================================
+ Coverage    84.18%   84.34%   +0.15%     
===========================================
  Files           78       76       -2     
  Lines         2586     2574      -12     
===========================================
- Hits          2177     2171       -6     
+ Misses         409      403       -6     

Flag	Coverage Δ
EditMode	31.04% <ø> (+0.14%)	⬆️
PlayModePlaythrough	66.20% <ø> (+0.30%)	⬆️
PlayModeScenes	60.17% <ø> (+0.04%)	⬆️
PlayModeScripts	60.68% <ø> (-0.07%)	⬇️

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