chore(tests): tie usage output to snapshots

We have another PR in flight that will start auto-generating some
content in `docs/content/commands`, and will run `postsnap`, which means
that if we tie the usage output to a snapshot, the docs will also be
updated when the snapshots change.

PR-URL: #3246
Credit: @wraithgar
Close: #3246
Reviewed-by: @isaacs

Author: wraithgar