It appears that if you define your own coding standard, then all classes, including abstract classes, in the /Sniffs directory that end with 'Sniff' are improperly instantiated when running phpcs.

There should be a check for whether a class is abstract before it is instantiated, even if its name ends in 'Sniff'. This could be useful if someone wants to maintain a naming convention in that namespace, but still make a class abstract (i.e. BaseMyStandardSniff) rather than changing it to BaseMyStandardHelper or something of the sort.