storage: assess state of current multi store test coverage

We document support for multi store (e.g. "run one node with multiple --store flags" https://www.cockroachlabs.com/docs/stable/recommended-production-settings.html#topology) and have large customers using it in production. However, feedback from Storage Engineering is that we may not be testing it as well as we should. We need someone to 1) see what multi store test coverage currently exists, 2) determine if additional test coverage is needed. The purpose of this issue is to track that work.

Jira issue: CRDB-29646

Begin by locating the existing multi-store tests in the CockroachDB repository and reviewing how they exercise multiple --store flags. Compare that coverage with the documented multi-store production topology, then record which scenarios are missing and whether additional tests are needed. Done means a clear coverage assessment and actionable test recommendations.

Written by the indexing model from the issue text.