Enhance tests to cover auto-completion

[talife]

Describe the bug

Following the issue #3840, it would be nice to have tests to make sure flux auto-completion works properly.

Steps to reproduce

N/A

Expected behavior

Every cobra cmd has been tested for auto-completion whenever possible

Screenshots and recordings

No response

OS / Distro

N/A

Flux version

N/A

Flux check

N/A

Git provider

No response

Container Registry provider

No response

Additional context

Would calling Execute on a command with some inputs and asserting the cmd OutOrStdout be a good start for this ?

Code of Conduct

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