test/integration: split PodGroup preemption tests into own package

The scheduler preemption integration package shares a flat 600s timeout
(KUBE_TIMEOUT in hack/make-rules/test-integration.sh) and had grown to
consume essentially all of it. When it went over, ktesting cancelled the
context mid-test and tore down the shared API server, so every remaining
subtest failed with connection refused. Whichever test happened to be
last in the package absorbed the failure regardless of its own cost.

The PodGroup preemption tests account for roughly half the runtime:

  TestPodGroupPreemption                          166.5s
  TestCompositePodGroupPreemption                 100.6s
  TestPodGroupPreemption_NominatedNodeNameRespected 7.0s
  TestPodGroupCycleStatePreserved                   4.1s
  TestPodGroupPreemptionStatus                      3.7s

They are self-contained, sharing no symbols with the rest of the package
in either direction, so move them to a subpackage that gets its own
timeout budget. This follows the existing preemption/misc and
preemption/nominatednodename layout.

No test logic changes.