test/py: fix NameError exception if bdi cmd is not supported

test/py raises an error, if a board has not enabled bdi command

>           pytest.skip('bdinfo command not supported')
E           NameError: global name 'pytest' is not defined

import pytest in test/py/u_boot_utils.py fixes this.
Signed-off-by: NHeiko Schocher <hs@denx.de>
Reviewed-by: NStephen Warren <swarren@nvidia.com>

test/py/u_boot_utils.py

@@ -10,6 +10,7 @@ import os.path
 import pytest
 import sys
 import time
+import pytest
 
 def md5sum_data(data):
     """Calculate the MD5 hash of some data.