Re-introduce db_load_balancing_initializer_tests without forking

What does this MR do and why?

Adds test (with some stubbing) to test for Database Load balancer initialization

Not terribly realistic but provides some insurance. I have removed Process.fork due to #333184 (comment 1081658113)

See also !96141 (merged)

Related issue: #333184 (closed)

Screenshots or screen recordings

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

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.