Smoke tests should use linkcheck config from book.toml

[wonderfulspam]

While expanding the test suite for #46, it occurred to me that the smoke tests should use the linkcheck config specified in the book.toml (example). Currently, the configuration is hardcoded for all tests and the linkcheck part of book.toml exists for cosmetic reasons only as far as I can tell.

I'm happy to submit a PR to fix this but wanted to get your thoughts first. My initial thought was to deserialize a mdbook-linkcheck Config struct from the relevant part of the book.toml but it seems like this could be fixed simply by not overriding the linkcheck config?