“Testy” is the Sundered name for a human attempt to weaponize the deadly water. 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.