* Created regression tests for the table of contents
* Refactoring to make the test more readable
* Fixed some bitrot and removed the (now redundant) tests/helper module
* Removed the include_str!() stuff and use just the dummy book for testing
* Regression tests now pass again!
* Pinned a `*` dependency to use a particular version
* Made sure test mocks return errors instead of panicking
* Addressed the rest of @budziq's review
* Replaced a file open/read with file_to_string
