R1:5ab14d08793f

Provide isolated, read/write storage fixtures for unit tests

Summary:
- Unit tests can request storage fixtures.
- We build one fixture across all tests in the process, which can quickstart (takes roughly 1s to build, 200ms to destroy for me). This is a one-time cost for running an arbitrary number of fixture-based tests.
- We isolate all the connections inside transactions for each test, so individual tests don't affect one another.

Test Plan: Ran unit tests, which cover the important properties…