Expose binding of an assertj-core dependency automatically to all "touched" test tasks

Associating the AssertJ dependency is done via the `test` configuration, i.e:

```gradle
dependencies {
// https://mvnrepository.com/artifact/org.assertj/assertj-core
testCompile group: 'org.assertj', name: 'assertj-core', version: '3.8.0'
}
```

It would be great to be able to assign this _once_ and have it assigned to all targets that have generated sources. Probably via a "global scope" property _or_ via the `assertJ` configuration.