Seems that none of the selectors are working, but have confirmed they work in the vanilla 1.12-pre5 snapshot.

 

Below are some commands and their output in chat (nothing in console)

/say @s : Selector '@s' found nothing

/say @p : Selector '@p' found nothing

/say @e : Selector '@e' found nothing

/say @a : Selector '@a' found nothing

/testfor @e : 

/testfor @s : Entity '@s' cannot be found

 /testfor @p : Entity '@s' cannot be found

/testfor @a : 

 

It seems the selectors do not work in any of the commands, have also tested using /advancement grant/revoke