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.
The first time we're asked to generate a preview we'll generate one of the maximum dimension indicated in the configuration and all future resizing requests will be done on that preview in order to not waste time converting the same file over and over.
One of the fixes required for #12465
