This sprinkles track_caller on some more test functions to give more
useful line numbers on errors when a test fails.
read_to_string was changed since it couldn't track caller on a closure.
This fixes an issue where the `check` methods would inadvertently
rebuild the books if the `mdbook build` command is run. Normally this
isn't too much of an issue unless the `build` command is run with
options that affect the output.
This updates everything for the move of utils to mdbook-core. There will
be followup commits that will be moving and refactoring these utils.
This simply moves them over unchanged (except visibility).
