Test full kernel-to-user and user-to-kernel path management code paths

[ossama-othman]

Existing tests focus predominantly on individual mptcpd components, without much kernel involvement. Tests should be written or improved to cover the full path management code path, including:

• Kernel to user space
  • Trigger path management events in the kernel so that they are handled by mptcpd, and eventually routed to a mptcpd plugin.
• User to kernel space
  • Issue path management commands through a mptcpd plugin, and very that the commands were handled by the kernel.