libct/int/execin_tty: help debug a flake

This helps to debug https://github.com/opencontainers/runc/issues/2425

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/integration/execin_test.go

@@ -66,7 +66,7 @@ func TestExecIn(t *testing.T) {
 		t.Fatalf("unexpected running process, output %q", out)
 	}
 	if strings.Contains(out, "\r") {
-		t.Fatalf("unexpected carriage-return in output")
+		t.Fatalf("unexpected carriage-return in output %q", out)
 	}
 }