test: disable ephemeral volume tests on Kubernetes <= 1.15

The feature was alpha then and seems to have issues with waiting for
NodeUnpublishVolume to complete when deleting a pod, which triggers
our volume leak detection and causes test failures.

test/test.make

@@ -147,6 +147,24 @@ TEST_E2E_SKIP_ALL += should.access.volume.from.different.nodes
 TEST_E2E_SKIP_1.14 += volumeMode.should.not.mount.*map.unused.volumes.in.a.pod
 TEST_E2E_SKIP_1.15 += volumeMode.should.not.mount.*map.unused.volumes.in.a.pod
 
+# It looks like Kubernetes <= 1.15 does not wait for
+# NodeUnpublishVolume to complete before deleting the pod:
+#
+# Apr 21 17:33:12.743: INFO: Wait up to 5m0s for pod "dax-volume-test" to be fully deleted
+# pmem-csi-node-4dsmr/[email protected]: I0421 17:33:34.491659       1 tracing.go:19] GRPC call: /csi.v1.Node/NodeGetCapabilities
+# pmem-csi-node-4dsmr/[email protected]: I0421 17:33:45.549013       1 tracing.go:19] GRPC call: /csi.v1.Node/NodeUnpublishVolume
+# pmem-csi-node-4dsmr/[email protected]: I0421 17:33:45.549189       1 nodeserver.go:295] NodeUnpublishVolume: unmount /var/lib/kubelet/pods/1c5f1fec-b08b-4264-8c55-40a22c1b3d16/volumes/kubernetes.io~csi/vol1/mount
+# STEP: delete the pod
+# Apr 21 17:33:46.769: INFO: Waiting for pod dax-volume-test to disappear
+# Apr 21 17:33:46.775: INFO: Pod dax-volume-test no longer exists
+#
+# That breaks our volume leak detection because the test continues
+# before the volume is truly removed. As a workaround, we disable
+# ephemeral volume tests on Kubernetes <= 1.15. That's okay because the feature
+# was alpha in those releases and shouldn't be used.
+TEST_E2E_SKIP_1.14 += Testpattern:.Ephemeral-volume Testpattern:.inline.ephemeral.CSI.volume
+TEST_E2E_SKIP_1.15 += Testpattern:.Ephemeral-volume Testpattern:.inline.ephemeral.CSI.volume
+
 # Add all Kubernetes version-specific suppressions.
 TEST_E2E_SKIP_ALL += $(TEST_E2E_SKIP_$(shell cat _work/$(CLUSTER)/kubernetes.version))