How do they prevent icing when they cool the air?
The tests they're currently running are of the pre-coolers, which have a proprietary design to prevent icing. If it works, they will have overcome the biggest hurdle to building a hybrid scramjet/rocket engine.
Icing occurs around plane parts where air at ambient temperature is further cooled, for example by pressure drop. As it is the case in throttle body of piston engine.
Here incomming air is highly compressed -- and thus heated up -- due to impacting plane parts at hypersonic velocities.
The problem is that at high mach numbers, the incoming air is too hot to successfully combust. They have to chill it (using the cryogenic fuel), and chilling it would usually result in icing.
The whole point of these tests is to prove that their precooler system resists icing.