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

• PR has a meaningful title
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• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge and is not Work in Progress.
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• Breaking changes
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  • Documentation needed
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• User-facing changes
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  • Documentation needed
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• Testing - New and feature
  • 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
• Tooling
  • I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.
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