test: Move AZ spread e2e test to serial
test: Move AZ spread e2e test to serial

The AZ spreading tests depend on scheduler priorities, not predicates,
and thus a mostly full cluster can result in spreading being ignored
(scheduler prefers to schedule pods that violate spreading priority
if one or more nodes is full). By moving to serial, we ensure that
nodes have the necessary capacity. These tests were flaking fairly
frequently.

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