selftests: mptcp: functional tests for the userspace PM type

This change adds a selftest script that performs a comprehensive
behavioral/functional test of all userspace PM capabilities by exercising
all the newly added APIs and changes to support said capabilities.

Acked-by: Paolo Abeni <[email protected]>
Signed-off-by: Kishen Maloor <[email protected]>
Signed-off-by: Mat Martineau <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: kmaloor