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.

staging/src/k8s.io/apiserver/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",