The testfor command won't detect the Glowing tag for entities. This happens for all entities including mobs and players.

The command I used was:
/testfor @a

I tested to make sure I was using the right syntax by using:
/testfor @a

I assumed the Glowing tag was supposed to detect if the entity has the Glowing effect, given by a spectral arrow or by commands. It just comes up with the error:
[06:13:29] <player> did not match the required data structure

The screenshot shows me using the command in chat, but it behaves the same using command blocks too.