Consolidate test object creation under the wrappers pkg

[ahg-g]

In a few places in tests we create pools, models and pods. We used at the wrapper pattern in a few projects and it worked generally well. I think we can follow the same pattern here.

#350 introduces the wrapper pkg and adds the PodWrapper, we can follow up with InferencePoolWrapper, InferenceModelWrapper, PodMetricsWrapper and RequestWrapper (for which we have some functions that we can repurpose https://github.com/kubernetes-sigs/gateway-api-inference-extension/blob/main/pkg/ext-proc/test/utils.go#L63).