Test error response for invalid children
Validate WP children

Originially, this method was defined in a WP patch in OpenProject Backlogs.
Move WP move test to spec and update its specification

With the additional validation of a WP's children, a move of a WP to another
project is only valid if cross project relations are allowed. The old test
didn't consider that setting and just assumed that a WP move is valid across
project borders in any case (even if cross project relations are forbidden).
Adapt cukes to child WP validation
Merge pull request #2315 from opf/fix/17318_error_message_on_invalid_parent

17318 error message on invalid parent