test/py: Fix pytest4 deprecation warnings

Fix the following spit from pytest:

u-boot/test/py/conftest.py:438: RemovedInPytest4Warning: MarkInfo objects are deprecated as they contain merged marks which are hard to deal with correctly.
  Please use node.get_closest_marker(name) or node.iter_markers(name).
  Docs: https://docs.pytest.org/en/latest/mark.html#updating-code


    for board in mark.args:

In both cases, the later suggestion is applicable.

Reviewed-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Igor Opaniuk <igor.opaniuk@gmail.com>
[trini: Update for current file with a few more cases, un-pin pytest in CI]
Tested-by: Simon Glass <sjg@chromium.org> [on sandbox]
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Rini <trini@konsulko.com>