delete etcd socket file for unit tests

This change clean up the environment for etcd3 unit test.
Without this change, "make test" will leave some socket files in
workspace. And these socket files make hack/verify-generated-protobuf.sh
fails.

Kubernetes-commit: 4e9dcf3da536a9b956eb092fe9dbc5b2081cfb9d

pkg/storage/etcd3/store_test.go

@@ -692,6 +692,7 @@ func testPropogateStore(ctx context.Context, t *testing.T, store *store, obj *ex
 func TestPrefix(t *testing.T) {
 	codec := apitesting.TestCodec(codecs, examplev1.SchemeGroupVersion)
 	cluster := integration.NewClusterV3(t, &integration.ClusterConfig{Size: 1})
+	defer cluster.Terminate(t)
 	transformer := prefixTransformer{prefix: []byte("test!")}
 	testcases := map[string]string{
 		"custom/prefix":     "/custom/prefix",