testnode: fix Computer.updateConfiguration call
updateConfiguration is a method to set the list of partitions, not to add partitions to an existing computer. The previous (slapos.core <1.5.0) implementation of slap proxy was incorrect and was adding more partitions each time it was called, but that was never the indented behavior.
Prepare the list of partitions and call updateConfiguration only once to set all partitions.
This should fix SLAPOS-SR-TEST that are still using erp5.util