Contribution description

This provides a test application for emcute based on the MQTT-SN layer for scapy I provided in #11822. Since it uses UDP sockets it can be run without root, but requires scapy nonetheless.

Attached to this PR are fixes that I found with these tests.

Fixes

• emcute: fix payload copy error for emcude_pub
  len is used with the memcpy() to copy the payload to tbuf. With a
  payload provided that is just long enough to fill tbuf, len += 6
  leads to the memcpy() overriding data after tbuf (e.g. the
  mutex that is unlocked right after) and thus resulting in potential
  segmentation faults.
  Additionally + 6 can only be applied if the total packet length is
  below 256 (see spec), so len + pos is what needs to be provided to the
  corresponding send functions instead (pos adapts to the header length
  of the PUBLISH message) (now in emcute: fix payload copy error for emcute_pub #11957)
• emcute: fix length field calculation
  The length field in an MQTT packet carries the total length of the
  packet. If it is below 256 (i.e. fits in one byte) only one byte is
  used for the length field. If it is larger than that 3 bytes are used,
  with the first byte having the value 0x01 and the remaining bytes
  representing the length in as a 2 byte unsigned integer in network byte
  order. Resulting from that it can be assessed that the check in
  emcutes's set_len() function is wrong as it needs to be checked if
  len is lesser or equal to 0xff - 1. len <= (0xff - 1) can be
  simplified to len < 0xff. For some larger packages this safes 2 bytes
  of wasted packet space. (now in emcute: fix length field calculation #11958)

Testing procedure

Make sure you have a recent version of scapy for python3 installed.

Run

BOARD="<your choice>" make -C all test

(the blacklisted boards are excluded of course).

Issues/PRs references

Requires on #11822.