Tests in JavaScript go beyond MAX_SAFE_INTEGER:

• last test of theharder tests block:

  Test.assertEquals(solution(200000000), 9333333166666668);
  // 9333333166666668 <-- expected
  // 9007199254740991 <-- MAX_SAFE_INTEGER
  

• RNG in the random tests:

const num = ~~(Math.random() * 200000000)