Add BF Required to Test Builds

Previously, the environment variable that ensures all message creators have
a `BeanFactory` required setting the `SI_FATAL_WHEN_NO_BEANFACTORY` environment
variable to `true`.

Add this variable to `build.gradle`, enforcing the test for all builds.


__Once merged, we can remove the env. variable from the CI build(s)__
Cool! I missed this Gradle ability when I had done a line above. Thanks, good to know.

From other side it is interesting what led you to do this change.
And I don't see any changes to the Framework to be sure that this change has place.

And one more point: I remember as once I missed `BF` property for some `Mock` in DSL tests. 
So, an OS ENV option helps here for any project which is based on SI.

Your turn?

Otherwise, of course, LGTM and it can be merged after your comments