Use real filesystem in ops.testing backend (canonical#960)

This pull request refactors the testing module of the Ops library,
switching its backend from an in-memory virtual filesystem to a
real filesystem for simulating filesystem operations within
containers. Furthermore, it exposes a new API,
`Harness.get_filesystem_root`, enabling developers to interact
with the simulated filesystem during test cases for both populating
and inspecting the testing container filesystem.