Test/add runtime manager tests

[miledxz]

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Proposed changes

Reopening pr trough another pr because issues on github

update on this: https://github.com/nginxinc/nginx-gateway-fabric/pull/1555, since I did mistake and discarded all commits, I haven't find better solution so I opened another pr @sjberman @kate-osborn

• [ x] I have read the CONTRIBUTING doc
• [ x] I have added tests that prove my fix is effective or that my feature works
• [ x] I have checked that all unit tests pass after adding my changes
• [ x] I have updated necessary documentation
• [ x] I have rebased my branch onto main
• [ x] I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes, please add a brief note that summarizes the change.