Prevent NRE in Unix computer cmdlets with test hooks active

[vexx32]

PR Summary

Test hooks being enabled prevent _process from being defined & causes the Dispose() method to throw a NRE on the Unix Computer cmdlets.

PR Context

Less errors please. Also #12482 for the broader issues in this file.

PR Checklist

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    • Issue filed:
• Testing - New and feature
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[ghost]

🎉v7.1.0-preview.4 has been released which incorporates this pull request.:tada:

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