* Prohibit 'print.md' files
Fix#258 by emmiting an error whenever an mdBook contains a
"print.md" file in its root.
* Apply suggested changes
* Add tests
* 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
Move non-test test module files into their own directories to prevent
cargo from running them as tests. Then suppress the left-over warnings.
Move *dummy book* code and data into a shared folder, and leave the rest
of helper utilities (one function) in the original module.
