Add test cases for CFI
user
```ruby
require_relative 'helper'
require 'epub/cfi'

class TestCFI < Test::Unit::TestCase
  def test_escape
    assert_equal '^^', EPUB::CFI.escape('^')
  end

  def test_unescape
    assert_equal '^', EPUB::CFI.unescape('^^')
  end
end
```
assistant