All-in-one in-memory client driver for testing

When writing unit tests for layers (or other FDB client usages), typically a test will run as if it is the sole user of the cluster, and tests, today, have to go to lengths to ensure that each test run uses a different key prefix, or clean up a fixed key space prior to running, to allow tests to isolate themselves from one another. This generally works well, but it is compounded by other cluster-wide data that takes more effort, such as the directory layer.

Given how the simulation framework works, it seems like it wouldn't be a huge stretch to have a special client driver (or a configuration of the existing driver), to run the cluster code within the driver itself such that, at minimum, the driver could connect to a "virtual" cluster that is configured to write to a directory that is unique to the test or, for bonus points, use a truly in-memory storage engine.

Start by studying the simulation framework and the existing client driver, which the issue identifies as the relevant starting points. Define what a virtual cluster must provide for isolated tests, including a unique directory and, if feasible, an in-memory storage engine; done means tests can connect without manual key-prefix isolation or cleanup.

Written by the indexing model from the issue text.