The test that Matt faces comes in Chapter Nineteen, which is when he is taken to Attean's village and joins in a series of different activities that are practised by Indians. One of these in particular is when they play a game together which is kind of like football but where the ball can only be hit back and forth with sticks. Note how Matt realises that he is undergoing a test at this stage:
The Indian boys were bewilderingly quick and skillful, and they wielded their bats with no heed for each other's heads, and certainly not for Matt's. It was no accident, he knew, when an elbow jabbed suddenly into his right eye. These boys were putting him to the test.
Matt is deliberately being tested because he is not Indian and a white boy. The way he responds by continuing to do his best and ignoring the jibes and prods ensures that he passes that test. The new closeness that Matt experiences with Attean's dog and the way that Attean leaves Matt to find his way back to his cabin for the first time demonstrates that Matt has passed this test.