feat(cmd/influx): add cli testing framework

[kelwang]

This PR is the CLI testing framework. and example of user subcommand.

• CHANGELOG.md updated with a link to the PR (not the Issue)
• Well-formatted commit messages
• Rebased/mergeable
• Tests pass
• http/swagger.yml updated (if modified Go structs or API)
• Documentation updated or issue created (provide link to issue/pr)
• Signed CLA (if not already signed)

[imogenkinsman]

Looks good!

What would be really awesome is if we did proper integration testing by running an in-memory influxdb (influxd --store=memory --e2e-testing). Because the http service is mocked, this is testing a fairly thin layer: our cobra configuration. I think 99% of our cli regressions will be caused by underlying changes to kv, which would only be caught by integration testing.

Thoughts?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically closed because it has not had recent activity. Please reopen if this issue is still important to you. Thank you for your contributions.