Is there a canonical "test SSN" that is used by convention, so is understood by all who see it that it is not a real SSN?
There are multiple number groups and some particular numbers that will never be allocated:
Consider using one of these (the obviously invalid 000-00-0000 would be a good one IMO).
(Answer has been updated to provide source information beyond Wikipedia and remove information that is no longer accurate after the SSA made its randomization change in mid 2011.)