Write automated tests for shell completions

### Is your feature request related to a problem? Please describe

We often receive reports of broken shell completions, and I've accidentally shipped bad changes to shell completions a couple times.

### Describe the proposed solution

I would like automated tests for our shell completions scripts as described here: https://github.com/asdf-vm/asdf/pull/1933#issuecomment-2650748621

### Describe similar `asdf` features and why they are not sufficient

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### Describe other workarounds you've considered

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Start with the proposed solution and discussion in the linked pull request comment, then locate the shell completion scripts in the repository and review how they are invoked. Determine the supported completion behaviors that need coverage; done means automated tests detect broken shell completions before release.

Written by the indexing model from the issue text.