Improve and unify testing facilities (#1844)

* Improve and unify testing facilities

This improves the testing facilities by making the test client easier to
use. It also removes code that is not required for the test client.
Besides that it also moves the test service and test client under
`node/test`.

* Update Cargo.lock

* Update node/test/client/src/block_builder.rs

Co-authored-by: Andronik Ordian <write@reusable.software>

* Remove explicit lifetime annotation

* Fix warnings and add extra `BlockBuilderExt`

Co-authored-by: Andronik Ordian <write@reusable.software>