Add support for NUnit3 XML-reports

[fflaten]

PR Summary

Add initial support for NUnit3-compatible XML-reports for test results.

Features:

• Support standard output
• Use UTC time (string format 'o')
• Distinguish Context vs Describe using a _TYPE property
  • Note: Only available during Invoke-Pester, not in Export/ConvertTo-NUnitReport
• Add tags as properties
  • Overkill? Related Nunit test parameters #1639.
• Add Data as properties (when Data/TestCases/ForEach is dictionary)
• Support site-attribute to identify BeforeAll/AfterAll vs Test-failure
• Support Assembly test-suite per testfile. Enables filter on test file in Azure DevOps etc.
  • Add duplicate environemnt-elements (one per assembly), but worth it
• Support auto-parameters for both parameterized blocks and tests
  • Ex. name="TestBlock(1,2,3)" when block or test name doesn't use variables
  • Note: Only supported when -ForEach/TestCases are used with dictionary-values

Limitations:

• name is still full path for test-case-elements.
  • Workaround because Reports/CIs like Azure DevOps doesn't use fullname-attribute as intended for NUnit3. See comment

TODO:

• Tested with Azure DevOps Publish Task (most/all features)
• Tested with ReportUnit
• Tested with other CIs? Which? Who can test?

PR Checklist

• PR has meaningful title
• Summary describes changes
• PR is ready to be merged
  • If not, use the arrow next to Create Pull Request to mark it as a draft. PR can be marked Ready for review when it's ready.
• Tests are added/update (if required)
• Documentation is updated/added (if required)
  • Will be updated post-release. Issue created Update Test Results docs with NUnit3 support docs#265

Fix #1087
Fix #1879
Fix #1284
Related #2039
Related #1530
Related #301
Related #2143

[fflaten]

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[fflaten]

I think this is finally ready for review and a experimental release.

I regret self-assigning this task. The only thing worse than no official schema is schemas that doesn't match docs nor the original product's output. 😄

[fflaten]

ReportUnit 1.5 seems to use name on suites also, so might need to use fullname as workaround there as well. Unlike test-case there isn't another attribute we can place the name is for those who need it, so waiting on feedback/requests before changing it.