Fix cluster service reindexing params

What does this MR do and why?

Related to #422938 (closed)

• ClusterReindexingService sends .reindex method with: from, to, max_slice, slice params
• The reindex method passes the params to ::Search::ReindexingService and adds wait_for_completion
• This MR fixes the param name used to populate the slice id as it didn't match what was being sent. I also added some failing specs first

Screenshots or screen recordings

N/A

How to set up and validate locally

1. enable elasticsearch in gdk
2. setup advanced search and index everything using the gitlab:elastic:index rake task
3. initiate a zero-downtime reindex from the Admin UI
4. zero-downtime reindex is run using a cron worker. you can test the process faster by running the cron worker manually after the reindexing process has started: ElasticClusterReindexingCronWorker.new.perform
5. verify the process finishes successfully (if you try the same steps on the master branch the reindex process will fail with the error described in the related issue)

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.