Actually the coop bots were CREATED in between you waking up GLaDOS and escaping Wheatley but were TESTED just after you escaped. The reason the plants were still there was because GLaDOS was still cleaning up from when she was dead AND Wheatley.
yes, that was GLaDOS' testing.
if you haven't noticed p-body in the test chamber yet, then you are slow.
that test chamber he tore apart was a real chamber that was being used by the co-op robots because Wheatley was failing to test them. that's what i hope the dlc is about.