KIO autotests: repair JobTest::moveDestAlreadyExistsAutoRename

Summary:
It failed for me, and for FreeBSD on CI.
I guess the Linux CI has it all on the same partition, so the moving
across partition case isn't really tested there.

See explanations in the code comment.

Test Plan: Now jobtest passes locally

Reviewers: meven, apol

Reviewed By: apol

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28184