changefeedccl: ensure multitenancy test coverage

Many data-inensive customers will be using shared process multi tenancy in 23.2. We need to ensure changefeeds are well tested in this environment. We need to practically:
1. Ensure we don’t disable multitenancy in any unit tests
2. Use multi tenant metaphorically in roachtests

Jira issue: CRDB-31386

Epic CRDB-9179

Start by locating the changefeedccl unit tests and roachtests referenced by the issue, then check where shared-process multitenancy is disabled or absent. Done means the relevant unit tests retain multitenancy coverage and roachtests use the multi-tenant setup described in the issue.

Written by the indexing model from the issue text.