Fix specs by initialising connection using `test` env

What does this MR do and why?

This MR forces Gitlab::Redis::Queues to call .ping, effectively initialising a new connection in the connection pool if empty. This prevents the staging rails env stub from initialising a Redis connection using the gem's defaults.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.