* Optionally use an on-disk sqlite db in tests

When debugging a test it is sometimes useful to inspect the state of the
DB. This is not easy when the db is in-memory: one cannot attach the
sqlite CLI to another process's DB.

With this change, if SYNAPSE_TEST_PERSIST_SQLITE_DB is set, we use
 `_trial_temp/test.db` as our sqlite database. One can then use 
`sqlite3 _trial_temp/test.db` and query to your heart's content.

The DB is destroyed and recreated between different test cases.

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>