splitting from the main issue (#765) to prevent polluting it

Currently, "standard" filesystems can indeed used for ev3dev /sys emulation. This is intended primarily for unit testing. The way it works is by creating the necessary files & folders usingfake_ev3dev helper classes and then relying on sensors reading their attributes as regular files from a mock sysfs directory. All sysfs structures should exist underCURRENT_ROOT_PATH; this is/sys/class on EV3 and a custom temporary directory when running unit tests.

Sysfs and real filesystem sometimes behave differently; this can be observed ontruncate()/write() behaviour and more. For now, these differences need to be worked around, as most of the current unit tests likely use the regular files to mock things.

The emulation accuracy could theoretically be improved by implementing a custom filesystem throughFUSE. That would allow implementing special handling of the individual syscalls to more closely follow the behaviour of real /sys. However, while it looks interesting, I'm not sure there's much to be gained by implementing that.

@dwalend@jabrena