Add code coverage for DataGridViewImageCell

[Zheng-Li01]

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

Proposed changes

Add unit test file:DataGridViewImageCellTests.cs for public/protected methods/properties of the DataGridViewImageCell.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 76.79330%. Comparing base (ece6f6d) to head (8b518fd). Report is 51 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13616         +/-   ##
===================================================
+ Coverage   76.58723%   76.79330%   +0.20606%     
===================================================
  Files           3253        3257          +4     
  Lines         640992      642349       +1357     
  Branches       47439       47561        +122     
===================================================
+ Hits          490918      493281       +2363     
+ Misses        146541      145406       -1135     
- Partials        3533        3662        +129     

Flag	Coverage Δ
Debug	76.79330% <100.00000%> (+0.20606%)	:arrow_up:
integration	18.98375% <ø> (+0.50159%)	:arrow_up:
production	51.37602% <ø> (+0.35426%)	:arrow_up:
test	97.41905% <100.00000%> (+0.00730%)	:arrow_up:
unit	48.76126% <ø> (+0.32932%)	:arrow_up:

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