Make it easier to specify the chapter path in `mdbook test`

[ehuss]

The --chapter CLI option for mdbook test can be confusing to use when using file paths. It requires a path relative to the source directory. However, one typically is running mdbook from the root of the book directory. When using tab completion in a shell, this ends up with a path like src/chapter_1.md which does not work.

I'm not sure, but I think it would be helpful if --chapter accepted paths that are either relative from the summary, or relative from the book root. In practice it shouldn't be ambiguous in almost all cases.

This would be helpful to avoid confusion, and to make it easier to use the option.