test/py/conftest.py: Replace get_marker() with iter_markers()

As per https://docs.pytest.org/en/latest/mark.html#updating-code making
use of node.get_marker() has been deprecated and needs to be replaced,
in our case, with node.iter_markers().

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
1 file changed