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conncheck: add a self test function

This function tests:
 * if priorities are consistent in the check list
 * if local and remote candidates are distinct for each pair
 * if local and remote candidates transports are consistent
 * if local, remote and pair socket pointer are consistent
 * if local candidates have distinct address and base address
 * if candidates transport and sockets types are compatible
 * if the check list if sorted according to priorities
 * if the stun retransmit flag is set consistently wrt the nominated pair
 * if the selected pair is the first nominated pair in the checklist
 * if pairs in state waiting and frozen per foundation are compatible
   with RFC8445
 * if local candidates have different port numbers
 * if stun transaction list matches with in-progress pair state
 * if stun transaction list matches with the retransmit flag
 * if component state is compatible with the number and type of the pairs
 * if remote candidate transport is compatible with the nomination mode
   and controlling role
 * if the number of nominated pairs is compatible with the nomination
   mode and controlling role