[PlacesItemModelTest] Ignore user tag places

Instead of listing the user tags (as tried in D21017), the test should not have
access to the user's baloo DB in the first place.

To do so, we use a QTemporaryDir as $HOME for the test process and for
its ioslave children (by setting KDE_FORK_SLAVES=yes).

While at it, drop the no-op removeTestUserData() function.