Don't worry about it, you are just catching up. It has been standard practice in the operational side of the oil industry for years,i.e. refineries, Shipping etc. I have been to some offshore oil rigs and had to take a breathaliser to get on the helicopter ( not sure about the pilot)..
Where i worked an independent test agency arrived at a time of their own discretion and randomly select ten percent of the people on site with no exceptions. It doesn't cause any stress to take the test.I suppose if you were concerned that you would fail it would be stressful but failure is very rare, all employees know the rules and virtually all them abide by them..