py: tests: Mark mii tests as xfail

On systems which have mii commands enabled but there is no ethernet
controller these commands fails. For example zcu100, sanbox.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

test/py/tests/test_mii.py

@@ -4,12 +4,14 @@
 
 import pytest
 
+@pytest.mark.xfail
 @pytest.mark.buildconfigspec("cmd_mii")
 def test_mii_info(u_boot_console):
     expected_response = "PHY"
     response = u_boot_console.run_command("mii info")
     assert(expected_response in response)
 
+@pytest.mark.xfail
 @pytest.mark.buildconfigspec("cmd_mii")
 def test_mii_list(u_boot_console):
     expected_response = "Current device"