Implement a memory backend as a mock to support test cases

[amboar]

Currently the tests have to invoke a lot of machinery to provide mocks for the various character devices. This strategy was introduced in the early days where the codebase was missing a number of the abstractions that have come to life over time. We can eliminate the need to mock the mtd backend for test cases that are not exercising it (e.g. the protocol-level test cases).