Store persistent state for devices in onidriver_test

[bparks13]

Making changes to the Enable register for test devices has no effect as they only update during a reset. Resetting in this context means restarting the whole program, which overwrites any changes made to the registers anyway.

It would be good to store a persistent state for test devices that can be loaded by oni-repl and applied, mimicking real world hardware via a virtual state. For Linux, this configuration could be placed in /tmp, but I don't know the best place to store it for Windows installations.

[jonnew]

For the record, Test0 devices that exist in actual hardware (like those used for debug during FPGA development) do store persistent state. What we want is for the onidriver_test's internal Test0 devices to have this behavior.