Migrate tests to v6 assertions

[fflaten]

PR Summary

Start migrating tests in the repo to Should-* assertions.

Not covered:

• Tests in assert and assertions folders which test themselves and should use Verify-*.
• Should -Not -Throw. I like the errors with -Because 🙂
• Should -Exist. No equivalent and I like the path-specific error message.
• Should -Invoke. Should-Invoke syntax has own tests, but not Should -Invoke which relies on Mock.Tests.ps1

PR Checklist

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• Summary describes changes
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• Tests are added/update (if required)
• Documentation is updated/added (if required)

[fflaten]

Identified a few issues during this dogfooding: #2872, #2873 and #2874 + notes in not covered section above

Pester.Tests.ps1 should also be cleaned up. PSv3 guards and use of script variables (= test order dependency) #2877