Implement tiered storage multi-database test

We have a working POC and we should be able to write (or extend current) tests to have them use the same bucket local_store bucket directory but with different prefixes. A single test with multiple connections to different database home directories can specify different prefixes but use the same names for tables but with different data.

Some test verifications would be to make sure the cache and contents are what we expect at all times, calling flush_tier and/or checkpoint and verifying content. We want to make sure there are no conflicts sharing the bucket or the bucket cache.