Configure post-submit test for HNC

Note: this changed has not been tested, I don't know how to test it. If
this change break then post-submit tests for HNC will simply not run,
but developers can still merge in their changes because it's not
supposed to affect pre-submit tests.

config/jobs/kubernetes-sigs/wg-multi-tenancy/hnc-postsubmit.yaml

@@ -0,0 +1,28 @@
+postsubmits: 
+  - name: pull-hnc-test
+    always_run: false
+    decorate: true
+    decoration_config:
+      timeout: 1h
+    max_concurrency: 5
+    path_alias: sigs.k8s.io/multi-tenancy
+    run_if_changed: "incubator/hnc/.*"
+    labels:
+      preset-kind-volume-mounts: "true"
+      preset-dind-enabled: "true"
+    spec:
+      containers:
+      - image: golang:1.14
+        command:
+          - ./incubator/hnc/hack/post-submit-test.sh
+        # docker-in-docker needs privileged mode
+        securityContext:
+          privileged: true
+        env:
+        - name: REPO_INFO
+          value: https://github.com/kubernetes-sigs/multi-tenancy
+        - name: K8S_VERSION
+          value: v1.15.11@sha256:6cc31f3533deb138792db2c7d1ffc36f7456a06f1db5556ad3b6927641016f50
+    annotations:
+      testgrid-dashboards: wg-multi-tenancy-hnc
+      testgrid-tab-name: postsubmit-tests