selftests: mptcp: Add tests for userspace PM type

selftests: mptcp: Add tests for userspace PM type

These tests ensure that the in-kernel path manager is bypassed when
the userspace path manager is configured. Kernel code is still
responsible for ADD_ADDR echo, so also make sure that's working.

Tested-by: Geliang Tang <[email protected]>
Acked-by: Paolo Abeni <[email protected]>
Co-developed-by: Matthieu Baerts <[email protected]>
Signed-off-by: Matthieu Baerts <[email protected]>
Signed-off-by: Mat Martineau <[email protected]>
Signed-off-by: Jakub Kicinski <[email protected]>