Why would Pirelli schedule a rain tire test in Bahrain except that they have recent test data from the new car specs on the Bahrain track.
They had planned to spray water on the track via sprinklers — it would be a controlled amount of water to simulate light, medium and heavy rain (there are two types of rain tires one for light conditions and the other called: full wet).
There are new vehicle specifications this year and the rain tires are new and untried...
Hope this helps!