tests: 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: Heiko Schocher <hs@denx.de>
Reviewed-by: Stephen Warren <swarren@nvidia.com>