Force the user to choose a backend if she runs tests. (#2902)
Force the user to choose a backend if she runs tests. (#2902)

* Use maven-enforcer to ensure a test plugin has been chosen

Fixes #2839

* Do not enforce a test backend choice if tests are being skipped.

crockpotveggies/deeplearning4j,crockpotveggies/deeplearning4j,crockpotveggies/deeplearning4j,crockpotveggies/deeplearning4j,crockpotveggies/deeplearning4j,crockpotveggies/deeplearning4j