python3: skip test_multiprocessing/test_active_children test

This test is causing problems on the Autobuilder, so disable it for now.

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit ac000b00)
Signed-off-by: Steve Sakoman <steve@sakoman.com>