tests: Ensure files are in backend dirs

[robin-nitrokey]

The run methods used by the tests should ensure that all created files are inside the global and client backend directories (probably with an exemption for the RNG state).

[sosthene-nitrokey]

I tested this manually with DebugDumpStore. Maybe this syscall should actually return a structure with the filesystem described so that it can be used in test (behind a flag and using HashMaps to have no size limitation).

[robin-nitrokey]

As we have access to the platform, we can also access the store directly.