selftests: mptcp: add disconnect tests

Performs several disconnect/reconnect on the same socket,
ensuring the overall transfer is succesful.

The new test leverages ioctl(SIOCOUTQ) to ensure all the
pending data is acked before disconnecting.

Additionally order alphabetically the test program arguments list
for better maintainability.

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