Make ValidationPool accepts execution mode to run custom command or in process validation (#1622)

* Initial commit

Forked at: bf7ccb84
Parent branch: origin/master

* Propagate test mode all the way down to ValidationPool

* Update validation/src/validation_service/mod.rs

* Fix test

* WIP

Forked at: bf7ccb84


Parent branch: origin/master

* Update service/src/lib.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Adapt code to review suggestions

* Run validation inside the same process

* Add test

* CLEANUP

Forked at: bf7ccb84


Parent branch: origin/master

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>