Refactor shared data tests so they don't require root.

We now specifically test the use cases of writing data between greeters and sessions.
The new tests do not check the directory is removed when a user is removed (it's done in the background so we can't reliably be sure when it will complete).
The new tests do not check the ownership of the directories. Fixes: https://bugs.launchpad.net/bugs/1282289.

Approved by PS Jenkins bot, Robert Ancell.