Add test output management command

[gregorjerse]

The command is not finished. There are some issues to resolve.

• some connectors (S3...) store only files and not directories, so directory tests will fail.
• just by iterating through schema it is impossible to tell if reference is a directory and what should that directory contain.
• I could consult the database to check if the entry is a directory or not. If it is I could also obtain the list of all entries in that directory. But let stick with basic file testing for now.

[gregorjerse]

We already have validate_data method on the StorageLocation object that checks the existance of every referenced path in the given storage location.

Do we still need this check? It has some benefits (checks that the outputs are really there). If yes, I suggest we simplify it and only chech that the outputs of the data objects are referenced in the database. The health of the storage locations can then be checked with the validate_data if necessary.