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)__
NP; this came up in hip chat a few weeks ago (a missing BF in an XD test) and @pperalta (and others) suggested it would be nice to have it integrated in the build so devs didn't have to set it locally.

I tested it with both `test` and `testAll` tasks by commenting out line `65` in `AutoCreateDirectoryTests`. I also verified with the debugger that this is __added__ to the existing environment (rather than replacing it).

It came up again today (https://github.com/garyrussell/spring-xd/commit/58723ae29b8dc03e7b9c95e082b7bd6fe9a8f72f#commitcomment-8601128).

See https://github.com/spring-projects/spring-xd/pull/1293 which sets the var in XD.

Finally, it is not critical if this is missed by a user that creates components himself, that's why I didn't want to make it a hard error - especially for backwards compatibility with user code.

Of course, if they don't provide a bean factory in that case, (s)he won't use any custom message builder and (s)he won't have access to any custom configuration of the `IntegrationEvaluationContextFactoryBean` - these two are the main driver for providing a BF to every SI component.