Remove unneeded inheritance in InstallHelper.cs

[alyssa1303]

PR Summary

This PR removes inheritance of the class InstallHelper from PSCmdlet. This PR also fixes tests in InstallPSResource, SavePSResource, and UpdatePSResource to make sure the errors thrown are matched.

PR Context

Resolves #767

PR Checklist

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• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
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• Breaking changes
  • None
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  • Documentation needed
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• User-facing changes
  • Not Applicable
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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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[PaulHigin]

LGTM