Fix deprecation warning in test_multiprocessing

tests/functional/v1/test_multiprocessing uses psutil to get a list of
processes. The Processor class now uses children() instead of
get_children(); update the test with the current call to eliminate
the deprecation warning.

Change-Id: I83a6bb72b5282d3ab2a3b8e338ee3dc8668d2d53
Merge "Fix deprecation warning in test_multiprocessing"