tests: k8s-cron-job: set runtimeClassName to kata

The cron-job test workload was missing `runtimeClassName: kata`, which
meant the cron job was not actually being executed under the Kata
runtime, defeating the purpose of the test.

Set it explicitly, consistent with the sibling `job.yaml` workload.

Signed-off-by: Fabiano Fidêncio <ffidencio@nvidia.com>

tests/integration/kubernetes/runtimeclass_workloads/cron-job.yaml

@@ -8,6 +8,7 @@ spec:
     spec:
       template:
         spec:
+          runtimeClassName: kata
           containers:
           - name: pi
             image: quay.io/prometheus/busybox:latest