mdbook test ignores the directory argument when the -L flag is passed before it

[cvybhu]

Our mdbook workflow in https://github.com/scylladb/scylla-rust-driver suddenly started failing.

The failing command was:

mdbook test -L target/debug/deps docs

It started complaining that it can't find SUMMARY.md in the directory <repository>/src
The book is in the directory <repository>/docs and that's why we pass the docs argument to guide it to the right place.

Swapping the order of the arguments fixes the problem:

mdbook test docs -L target/debug/deps

This looks like a bug in argument parsing.

Refs: scylladb/scylla-rust-driver#616

[ehuss]

Thanks for the heads up! I have posted #1959 with a fix and will publish a new release.