Fix 1541 "`-WhatIf` results in extra, unwanted output and a spurious error"

[o-l-a-v]

PR Summary

• Fixed check for whether -WhatIf was passed
• Don't throw if -WhatIf was passed if no packages to install
• Make sure tests exists and works
• Fix and surpress PSScriptAnalyzer warnings in tests I touced

PR Context

1541

PR Checklist

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