Resolve "Vet testing in `completion`"

General MR

Summary

• Refactored all tests into one test func "TestAllCompletions", left "TestFooCompletion" for future reference
• Added powershell into the list of valid shells, added corresponding comments to load powershell completions for the session, and added corresponding test
• Removed all other unnecessary tests that were refactored into "TestAllCompletions"
• Updated Bash completion to V2, as the previous version was legacy

Relevant logs/screenshots

Linked Issue

Issue 68

Upgrade Notices

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Closes #68 (closed)