Added tests for distance method for linked lists.

 def test_distance_returns_distance_between_two_nodes
    list.push("hello")
    list.push("pizza")
    list.push("world")
    list.push("today")
    list.push("tomorrow")
    assert_equal 3, list.distance("hello", "today")
    assert_equal 2, list.distance("pizza", "today")
    assert_equal 2, list.distance("hello", "world")
    assert_equal 4, list.distance("hello", "tomorrow")
    assert_equal 1, list.distance("world", "today")
  end