The test subject in Party Escort Bot = Beans' Enrichment Center Activity has little to no information known about it. Today we will try to find out who it is.
Theories
Chell
I don't know why, this idea of Chell being the Test Subject keeps popping in my head. It might be false, however, because of one of Spoiler Guy's quotes:
I just wanted to say that Wheatley Laboratories' Frankenturrets serve as Wheatley's replacement for Weighted Storage Cubes when he was missing on Test Subjects. Later, he was sent to space by GLaDOS after his many attempts to kill Chell.
Why would he mention Chell by name if he could simply refer to her as "your test subject"?
A Party Escort Beans Laboratories robot
It is possible that the test subject is a Party escort beans laboratories robot. However, this may be disproved by the fact that the Test Subject wasn't put in the "Robots" part in the Mini-Wiki, but this disproval may be disproved by the fact that the Test Subject IS NOT present on the Characters list.[1] (see below)
Who?
Since the test subject isn't present on the Party Escort Beans Enrichment Center Mini-Wiki's character list, it is possible to suppose that the Test Subject doesn't actually even have specific facts for himself, and that the player itself is the Test Subject, much like the Character from Antichamber. (Note that the Character from Antichamber does have a single fact, its height. Since the Party Escort Beans Enrichment Center activity is made out of text, the characters' heights are unknown.)