Add test command to Shell completions

[muzimuzhi]

test command was supported since v3.2.0, see a8ffc31 (Add test command to run any unit test (#1092), 2023-07-28)

I hesitate to also add prepare command (see ./cmd/prepare.go@b43bd372) to completions, because it seems it's no longer an everyday command. The default completion command created by cobra is also not added, see my topic on the Exercism Forum.

[github-actions]

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