Why not take the three lines starting at [1] and move them in here, and then do away with `enable_desktop_fake_sdcard`? Is it a timing thing? Does the `set_char_pref` have to happen earlier in the life cycle of the test? If not, why not just move that code in here, and only execute it if the `self._device_root` isn't yet set? That would still allow the code to only be executed once, but will remove the need to manually call `enable_desktop_fake_sdcard`.

[1] https://github.com/mozilla-b2g/gaia/pull/19155/files#diff-6d544bbb7c759ffb68f344f348f7740eR420