You have to talk to Irrelevant Man to get warped to Shibu's Farm.
There are 2 kinds of mushrooms on the farm that you can collect, Orange Net Mushrooms and Orange Gooey Mushrooms.
1 Orange Net Mushroom = 3 points
1 Orange Gooey Mushroom = 1 point
You need at least 25 points to pass the test. When you collected enough, you have to go back to the Thief Guide and talk to Test Grader Comrade Brad.
Notice you have to die in order to get out of the farm.
Tips: You may want to take off your weapon to attack faster.