Add test coverage for DateTimePicker

[John-Qiao]

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

Proposed changes

Add unit tests for DateTimePicker's property and event

• Properties:

1. CalendarTitleBackColor

2. CalendarForeColor

3. CalendarTitleForeColor

4. CalendarTrailingForeColor

5. CalendarMonthBackground

• Events:

1. BackColorChanged

2. BackgroundImageChanged

Customer Impact

• None

Regression?

• No

Risk

• Minimal

Test methodology

• Unit tests

Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

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

Project coverage is 73.23479%. Comparing base (bd97476) to head (309d767). Report is 49 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #11033         +/-   ##
===================================================
+ Coverage   73.17333%   73.23479%   +0.06145%     
===================================================
  Files           3097        3096          -1     
  Lines         633955      633692        -263     
  Branches       47364       47336         -28     
===================================================
+ Hits          463886      464083        +197     
+ Misses        166522      166068        -454     
+ Partials        3547        3541          -6     

Flag	Coverage Δ
Debug	73.23479% <100.00000%> (+0.06145%)	:arrow_up:
integration	18.42694% <ø> (-0.02383%)	:arrow_down:
production	46.72867% <ø> (+0.05459%)	:arrow_up:
test	94.98054% <100.00000%> (+0.00600%)	:arrow_up:
unit	43.60773% <ø> (+0.06247%)	:arrow_up:

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[LeafShi1]

@lonitra Please help review this PR