Use dotnet test instead of dotnet xunit since the xunit runner has been deprecated

[bergmeister]

PR Summary

The dotnet xunit test runner has been deprecated and will not work with future version of .Net Core.
Replace it with dotnet test instead using the equivalent set of test options and add NuGet package for test adapter to still produce test results file in Xunit format.
Fix existing pre-test cleanup code that did not work as it was run BEFORE the working directory was set.
This was discussed/agreed in PR #7961 by @TravisEz13

PR Checklist

• PR has a meaningful title
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• Summarized changes
• Change is not breaking
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
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• User-facing changes
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  • User-facing Documentation needed
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• Testing - New and feature
  • Not Applicable or can only be tested interactively
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  • Make sure you've added a new test if existing tests do not effectively test the code changed
    • Add [feature] if the change is significant or affects feature tests