Add unit test case for DataGridView 7 events

[John-Qiao]

related https://github.com/dotnet/winforms/issues/10453

Proposed changes

• Add unit tests for below 7 events

1. BackColorChanged
2. BackgroundImageChanged
3. BackgroundImageLayoutChanged
4. ForeColorChanged
5. FontChanged
6. PaddingChanged
7. TextChanged

Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

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

Project coverage is 74.56167%. Comparing base (2a6733d) to head (74e8a72). Report is 13 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #11618         +/-   ##
===================================================
+ Coverage   74.55540%   74.56167%   +0.00627%     
===================================================
  Files           3040        3040                 
  Lines         629493      629605        +112     
  Branches       46835       46839          +4     
===================================================
+ Hits          469321      469444        +123     
+ Misses        156818      156800         -18     
- Partials        3354        3361          +7     

Flag	Coverage Δ
Debug	74.56167% <100.00000%> (+0.00627%)	:arrow_up:
integration	17.98157% <ø> (+0.01154%)	:arrow_up:
production	47.50930% <ø> (+0.00790%)	:arrow_up:
test	96.95225% <100.00000%> (-0.00222%)	:arrow_down:
unit	44.54530% <ø> (+0.02649%)	:arrow_up:

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[LeafShi1]

Using InlineData can make the code more concise， like this https://github.com/dotnet/winforms/blob/10f120ddca8dc60568e4b9cb93667e90746aade3/src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/SplitterTests.cs#L1063