Introduce the expectations file

Add an expectation.json file that contains the list of files used for flakes and skips as well as the baseline file.

When running a suite with a TOML file, the expectation for each entries of the suite are listed in the expectation file.

This is mainly used to ensure that a result directory can be easily matched with the correct expectation files that have be used for those results.

An optional name field is added in the SubConfig struct to allow finer matching when a suite contains a lot of entries.