The test uses KIdentityManagement::IdentityManager, so when run on local machine it reads
and writes into user's identities. This change forces fake XDG dirs to make sure we don't
touch user's stuff. Fake KDEHOME is needed to prevent config migration from user's KDEHOME
into the fake XDG dirs.

Also replaces some QVERIFYies with QCOMPAREs and adapts testCreateForward() test to match
with minor behaviour change in KMime and/or composer.