testDefaultCaption() {
    select.render(sandboxEl);
    const item1 = new MenuItem('item 1');
    select.addItem(item1);
    select.addItem(new MenuItem('item 2'));
    assertEquals(defaultCaption, select.getDefaultCaption());
    assertEquals(defaultCaption, select.getCaption());

    const newCaption = 'new caption';
    select.setDefaultCaption(newCaption);
    assertEquals(newCaption, select.getDefaultCaption());
    assertEquals(newCaption, select.getCaption());

    select.setSelectedItem(item1);
    assertNotEquals(newCaption, select.getCaption());

    select.setSelectedItem(null);
    assertEquals(newCaption, select.getCaption());
  }
Checks that the default caption passed in the constructor and in the setter
is returned by getDefaultCaption, and acts as a default caption, i.e. is
shown as a caption when no items are selected.