Recently introduced tmpfiles.d config file started creating
/var/tmp/taskotron-mockchain as root with 777. Mockchain tries to do
chmod on it and crashes (RHBZ 1209532). The solution is to add a
subdirectory and mockchain is now happy.
As a polish change, adjust group ownership of
/var/tmp/taskotron-mockchain. That does not work around the bug, but
seems more correct.