“Testy” is the Sundered name for a human attempt to weaponize the deadly water in the world of The Sundered. (Humans called Compound IG624). Though the main body of water flooding the planet kills if touched, it becomes harmless when removed (say, in a cup or bowl or bucket). These folks wanted it to retain its deadly properties even when separated.
Harry never learns much about how it works; suffice it to say that frightened human governments attempted to force their Sundered Ones to alter the very nature of the water.