Adds a few tests based on the suggestions in #2677

Tests should be self-explanatory from the commit message.

I've changed BookTest to use snapbox's Command to make use of its built in stdin - useful for testing commands like init and then giving answers to the prompts

This should cover the following from the suggestions:

• TOC fold tests
• {{#title}}
• init with git author configured
• init check contents of .gitignore
• Check that renderer or preprocessor uses PATH to locate mdbook-FOO

In addition, I've included:

• Tests verifying the generated 404 page, including the default, when a 404.md is provided, and when output.html.input-404 is configured
• Test verifying the behaviour when a theme folder already exists when using the init command