Replace '%s' in diff tests

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)

integration-cli/docker_cli_diff_test.go

@@ -85,7 +85,7 @@ func TestDiffEnsureOnlyKmsgAndPtmx(t *testing.T) {
 
 	for _, line := range strings.Split(out, "\n") {
 		if line != "" && !expected[line] {
-			t.Errorf("'%s' is shown in the diff but shouldn't", line)
+			t.Errorf("%q is shown in the diff but shouldn't", line)
 		}
 	}