The OP shouldn't trust the drug using coworker to do anything. The OP should go to someone higher than her supervisor and tell them why she is refusing to room with the drug user. This may well end with the OP being terminated because while the employer certainly can't force the OP to room with that coworker they can certainly fire her for not doing so and if she is smart she will prepare for that evenuality by having a new job available.
Just because a business test tests new hires it doesn't mean that they have to ever test them again and many if not most employers don't test because their insurance company or anyone else requires it so there is no barrel for them to be over.

