Fix nullability error CS8620 in tests

[am11]

Fix warning (as error):

src\winforms\src\System.Windows.Forms.Design\tests\UnitTests\System\Windows\Forms\Design\ToolStripContentPanelDesignerTests.cs(43,67): error CS8620: (NETCORE_ENGINEERING_TELEMETRY=Build) Argument of type 'string[]' cannot be used for parameter 'span' of type 'Span<string?>' in 'bool MemoryExtensions.Contains<string?>(Span<string?> span, string? value)' due to differences in the nullability of reference types.

This is to unblock dotnet/sdk#44843. I have tested with 10.0.100-alpha.1.24562.4 in global.json, this is the only instance of the error.

Microsoft Reviewers: Open in CodeFlow

[am11]

cc @ViktorHofer

[ViktorHofer]

Thanks a lot

[am11]

Basically it expects either both operands of Contains be string or string?. Mixing them confuses the new compiler due to the upcoming "first-class span" feature.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.73518%. Comparing base (8d8deef) to head (98c59e0).
Report is 4 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #12475         +/-   ##
===================================================
+ Coverage   75.71428%   75.73518%   +0.02089%     
===================================================
  Files           3152        3153          +1     
  Lines         635711      635797         +86     
  Branches       46970       46973          +3     
===================================================
+ Hits          481324      481522        +198     
+ Misses        150944      150841        -103     
+ Partials        3443        3434          -9     

Flag	Coverage Δ
Debug	75.73518% <100.00000%> (+0.02089%)	⬆️
integration	18.27004% <ø> (+0.01766%)	⬆️
production	49.31027% <ø> (+0.03946%)	⬆️
test	97.05103% <100.00000%> (+0.00072%)	⬆️
unit	46.28855% <ø> (+0.03312%)	⬆️

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