update test

Signed-off-by: Victor Vieux <vieux@docker.com>

integration-cli/docker_cli_rm_test.go

@@ -111,8 +111,10 @@ func TestRmContainerOrphaning(t *testing.T) {
 }
 
 func TestRmInvalidContainer(t *testing.T) {
-	if _, _, err := runCommandWithOutput(exec.Command(dockerBinary, "rm", "unknown")); err == nil {
+	if out, _, err := runCommandWithOutput(exec.Command(dockerBinary, "rm", "unknown")); err == nil {
 		t.Fatal("Expected error on rm unknown container, got none")
+	} else if !strings.Contains(out, "failed to remove one or more containers") {
+		t.Fatal("Expected output to contain 'failed to remove one or more containers', got %q", out)
 	}
 
 	logDone("rm - delete unknown container")