This is bad style; you should put the code inside the test where possible. In this case

test(function() {
  assert_true(document.getElementById('box'));
  //Note this code isn't run if the assert fails
  box.style.backgroundColor = '#00FF00';
  box.style.color = 'white';
  box.innerHTML = 'Pass';
  box.style.height = '100px';
  box.style.width = '100px';
});

Similar comments presumably apply to other tests.