Why do my test fail intermittently when running the whole suite?

One of my tests is failing intermittently when running the whole suite, but it doesn't fail when running it by itself.

I created a very basic repository with a vanilla application that reproduces the issue: http://ift.tt/2eb2ZRz.

Basically, I created a component with a text field and a button. The button is disabled while the text is empty.

The issues happens when two tests use the fillIn helper on the input.

Here is the testing code taken from the vanilla app:

test('test 1', function(assert) {
  visit('/');

  fillIn('input[type=text]', "Algo");

  andThen(function() {
    assert.equal(currentRouteName(), "index");
  });
});

test('test 2', function(assert) {
  visit('/');

  andThen(function() {
    assert.ok(find('input[type=submit]').is(':disabled'));
  });

  fillIn('input[type=text]', "Algo");

  andThen(function() {
    assert.ok(!find('input[type=submit]').is(':disabled'));
  });
});

As you can see test 1 only fills the input but doesn't do anything with it. The second test tests if the button is disabled.

Test 2 fails intermittently when running the whole suite. If you run ember test -s it fails, if you reload the browser tab (re running the whole suite without restarting the server process) it passes. The same behavior happens with multiple runs (one run fails, the next succeeds).

I didn't create a twiddle reproduction case because the test runner doesn't behave the same way.