test: enable tests using the default filesystem

Several (all?) of the volumeMode tests run with the default filesystem
(i.e. the empty string). If we don't list that as supported, raw block
tests are not executed:

S [SKIPPING] in Spec Setup (BeforeEach) [0.005 seconds]
PMEM Volumes
/nvme/gopath/src/github.com/intel/pmem-csi/test/e2e/storage/csi_volumes.go:57
  [Driver: pmem-csi]
  /nvme/gopath/src/github.com/intel/pmem-csi/test/e2e/storage/csi_volumes.go:102
    [Testpattern: Dynamic PV (block volmode)] volumes
    /nvme/gopath/src/github.com/intel/pmem-csi/vendor/k8s.io/kubernetes/test/e2e/storage/testsuites/base.go:92
      should store data [BeforeEach]
      /nvme/gopath/src/github.com/intel/pmem-csi/vendor/k8s.io/kubernetes/test/e2e/storage/testsuites/volumes.go:146

      Driver pmem-csi doesn't support  -- skipping

      /nvme/gopath/src/github.com/intel/pmem-csi/vendor/k8s.io/kubernetes/test/e2e/storage/testsuites/base.go:151

Author: pohly

test/e2e/storage/csi_volumes.go

@@ -64,7 +64,7 @@ var _ = Describe("PMEM Volumes", func() {
 					Name:        "pmem-csi",
 					MaxFileSize: testpatterns.FileSizeMedium,
 					SupportedFsType: sets.NewString(
-						"ext4", "xfs",
+						"", "ext4", "xfs",
 					),
 					Capabilities: map[testsuites.Capability]bool{
 						testsuites.CapPersistence: true,
@@ -74,6 +74,7 @@ var _ = Describe("PMEM Volumes", func() {
 					},
 				},
 				scManifest: map[string]string{
+					"":     "deploy/kubernetes-1.13/pmem-storageclass-ext4.yaml",
 					"ext4": "deploy/kubernetes-1.13/pmem-storageclass-ext4.yaml",
 					"xfs":  "deploy/kubernetes-1.13/pmem-storageclass-xfs.yaml",
 				},