Fix `mdbook test`

This fixes a problem where custom preprocessors were not being
registered when running tests. This was caused by the test function
rebuilding the preprocessor map.

This removes the code that was rebuilding the preprocessors and removing
the IndexPreprocessor when running tests. Skipping IndexPreprocessor was
added back in rust-lang#741 to fix
rust-lang#724 which was caused by
rust-lang#685 which added the
IndexPreprocessor.

Additionally, rust-lang#1986 added
running *all* preprocessors.

The IndexPreprocessor was removed because in the past the code was
testing against the source directly, and the path from `chapter.path` is
the converted `index.md` file, and that filename does not exist in the
source. This isn't a problem anymore because due to
rust-lang#891 it is not reading from the
`src` directory.

Note that this results in a minor change where the chapter path changes
from `README.md` to `index.md` in the output and the `--chapter` option.
I think I'm ok with that change, though it would be easy to switch it
back if that's an issue.