Replace originalStorage in tests with a proper teardown

Replace originalStorage in tests with a proper teardown

The purpose of $originalStorage in unit tests was to remount the old
root.
However that storage itself is already wrapped by storage wrapper, so
remounting it would rewrap the storage several times.

This fix makes use of "loginAsUser()" and "logout()" from the TestCase
class to properly initialize and cleanup the FS as expected.