Fix flaky SubmissionService test

[alexandrevryghem]

References

• Fixes Unit test redirectToEditItem is flaky #5553

Description

Fixed flaky SubmissionService test by using a stub class instead of a SpyObj to prevent the method to be spied on twice (during init & in the test where the value is explicitly set to a certain value).

Checklist

• My PR is created against the main branch of code (unless it is a backport or is fixing an issue specific to an older branch).
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[tdonohue]

👍 Thanks @alexandrevryghem ! This looks good to me. I also verified that this flakey code only exists on main, so there's no need to backport.