core: fix transaction destructiveness check once more

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.

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