Adjust unit tests to clarify existing device usage

Most of the unit tests run with check_mount=False, so if a device is
chosen by a test that was not created ahead of time, the test will still
work, since the "tmp" directory creation in DiskFile.create() using
mkdirs(), thereby creating the a directory for the device (perhaps that
should be fixed).

This change refactors the test code to use the exiting device
explicitly.

Change-Id: Ib4fbf069596672bb0b98f41573f312eabde38f98
Merge "Adjust unit tests to clarify existing device usage"