Update test files to use Pester 5.X

[alyssa1303]

PR Summary

This PR fixes test files to be compatible with Pester 5.X and updates the pipeline to use Pester 5.X.

PR Context

PR Checklist

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• [x] Summarized changes
• [x] Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
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• Breaking changes
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  • [ ] Documentation needed
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• User-facing changes
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  • [x] N/A 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
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[alerickson]

I think we also need to update ci.yml to version 5

[anamnavi]

I think we also need to update .ci/test.yml. (Pester is also referenced in ci_auto.yml, but it's referring to version 5.x there already)