Add the ability to specify dependencies for mdbook test to use when building

[Lokathor]

You apparently cannot currently specify a dependency to be used during mdbook test, but this is obviously extremely limiting in a language that tries to keep the stdlib minimal and push things into crates.

Perhaps mdbook could search for a Cargo.toml in the same folder as the book.toml or something like that, then use that dependency list.

[Lokathor]

whoops, i missed that this is an open issue, my mistake