The body doesn't build up a tolerance to test or any other androgen. Muscle cell ARs up regulate with the dosage over time. Gains slow down not because of desensitization but due to genetic limitations. If you put on 10 pounds in 2 months that is more than most people can gain in a year natty, so of course gains will slow down. Most people don't know how to train and diet properly to over come these plateaus. Also, when cycling the body will store more glycogen and nutrients in muscles. Once muscles reach a threshold were they are completely saturated no more nutrients can be stored. This is why muscles expansion slows down mid/end of cycle even though hypertrophy is continuing. You may gain 2 pounds or more a week the first 2 months on cycle, but then only a pound or less a week after that because once muscle cells are full of nutrients you will be only gaining tissue.