Add code coverage for DataGridViewImageColumn

[Zheng-Li01]

Related https://github.com/dotnet/winforms/issues/13442

Proposed changes

Add unit test file:DataGridViewImageColumnTests.cs for public methods of the DataGridViewImageColumn.cs

Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 76.63702%. Comparing base (5821493) to head (4fa6e28). Report is 107 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                  @@
##                main      #13547          +/-   ##
====================================================
+ Coverage   62.21083%   76.63702%   +14.42619%     
====================================================
  Files           3213        3254          +41     
  Lines         638302      641181        +2879     
  Branches       47201       47455         +254     
====================================================
+ Hits          397093      491382       +94289     
+ Misses        234153      146147       -88006     
+ Partials        7056        3652        -3404     

Flag	Coverage Δ
Debug	76.63702% <100.00000%> (+14.42619%)	:arrow_up:
integration	18.98721% <ø> (+0.19843%)	:arrow_up:
production	51.11800% <ø> (+32.32922%)	:arrow_up:
test	97.41310% <100.00000%> (+0.00898%)	:arrow_up:
unit	48.50076% <ø> (?)

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