scheduler_perf: create new pod immediately in "steady state"

The steady state pod creation test cases do not keep the scheduler 100% busy,
leading to a measured pod scheduling rate which is partly influence by how
quickly new pods get created. This is not necessarily bad because it measures
the whole "pod created -> claim created -> events observed -> scheduled" path,
but it is a bit unexpected.

Creating the next pod immediately after one pod got scheduled achieves better
parallelism between producer (tester) and consumer (scheduler), which is
visible as a higher scheduling rate. This is better because performance
improvements in the scheduler are more visible. Also, the state of the cluster
remains closer to the initial state and the code is simpler.

As this changes what is being measured, performance results are different. For
example, PerfScheduling/SteadyStateClusterResourceClaimTemplate/empty_500nodes
yields 64% higher SchedulingThroughput/Average.