Fix SPW delete test timeouts (see #11540).

This PR should eliminate omero.LockTimeout-s in the ``testScreen`` method. It increases the time needed for the delete operation to succeed by multiplying the initial scaling factor time by the amount of plates, wells, fields and acquisitions.

To test - verify that http://hudson.openmicroscopy.org.uk/view/Failing/job/OmeroJava-integration-develop/52/testngreports/ doesn't happen for a couple of days.