[cmdpal] Migrate some plugin's unit tests from PT run to cmdpal. (#40462)

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fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request
Migrate blow plugin's from UT to cmdpal:
1. TimeDate
2. WindowWalker
3. System
4. Registry

This PR is mostly helped by Copilot. Please feel free to change cases in
the future.

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## PR Checklist

- [x] **Closes:** #40461
- [x] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [x] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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Co-authored-by: Yu Leng <yuleng@microsoft.com>

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.TimeDate/Microsoft.CmdPal.Ext.TimeDate.csproj

@@ -11,6 +11,12 @@
     <ProjectPriFileName>Microsoft.CmdPal.Ext.TimeDate.pri</ProjectPriFileName>
   </PropertyGroup>
 
+  <ItemGroup>
+    <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleTo">
+      <_Parameter1>Microsoft.CmdPal.Ext.TimeDate.UnitTests</_Parameter1>
+    </AssemblyAttribute>
+  </ItemGroup>
+
   <ItemGroup>
     <EmbeddedResource Update="Properties\Resources.resx">
       <LastGenOutput>Resources.Designer.cs</LastGenOutput>