Update path of test\xUnit\xUnit.tests.csproj in PowerShell.sln

[markekraus]

PR Summary

In #8356 test\xUnit\xUnit.tests.csproj was moved to test\xUnit\xUnit.tests.csproj but PowerShell.sln was not updated. This is causing issues in Omnisharp in VS Code with loading the project. This PR updates the solution file to correct the new path and project name.

PR Context

Pain point while developing 😄

PR Checklist

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  • Documentation needed
    • Issue filed:
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[iSazonov]

@markekraus Thanks! I forget about sln.