The previous fix
e0312f4db "core: fix check for transaction
destructiveness" broke test-engine (noticed by Zbyszek).
Apparently I had a wrong idea of the intended semantics of --fail.
The manpage says the operation should fail if it "conflicts with a
pending job (more specifically: causes an already pending start job to
be reversed into a stop job or vice versa)".
So let's check job_type_is_conflicting, instead of !is_superset.
This makes both test-engine and TEST-03-JOBS pass again.