test/py: error message test_efi_selftest_device_tree

Correct the error message in test_efi_selftest_device_tree().

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/test/py/tests/test_efi_selftest.py b/test/py/tests/test_efi_selftest.py
index 07e4db0..516e41c 100644
--- a/test/py/tests/test_efi_selftest.py
+++ b/test/py/tests/test_efi_selftest.py
@@ -37,7 +37,7 @@
 	u_boot_console.run_command(cmd='bootefi selftest ${fdtcontroladdr}', wait_for_prompt=False)
 	m = u_boot_console.p.expect(['serial-number: Testing DT', 'U-Boot'])
 	if m != 0:
-		raise Exception('Reset failed in \'device tree\' test')
+		raise Exception('serial-number missing in device tree')
 	u_boot_console.restart_uboot();
 
 @pytest.mark.buildconfigspec('cmd_bootefi_selftest')