Add unit test for UpDownBaseDesigner

[Liv-Goh]

Related https://github.com/dotnet/winforms/issues/10773

Proposed changes

1. Add unit test UpDownBaseDesignerTests.cs for public properties and method of the UpDownBaseDesigner.
2. Enable nullability in UpDownBaseDesignerTests.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 97.03074%. Comparing base (572677b) to head (a47b22a). Report is 32 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                  @@
##                main      #12364          +/-   ##
====================================================
+ Coverage   75.60672%   97.03074%   +21.42402%     
====================================================
  Files           3115        1156        -1959     
  Lines         635138      351637      -283501     
  Branches       46921        5283       -41638     
====================================================
- Hits          480207      341196      -139011     
+ Misses        151479        9691      -141788     
+ Partials        3452         750        -2702     

Flag	Coverage Δ
Debug	97.03074% <100.00000%> (+21.42401%)	:arrow_up:
integration	?
production	?
test	97.03074% <100.00000%> (+0.00542%)	:arrow_up:
unit	?

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