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 <pabeni@redhat.com>
Signed-off-by: Kishen Maloor <kishen.maloor@intel.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>