Model test fixtures as a propert artifact Previously they were modeled as just a file collection, which confused the IDEA importer. They now use the more canonical way of defining an additional artifact.
In a next step they should be reimplemented using variant aware dependency resolution, so that we don't need a special DSL to declare these dependencies.
Fix Gradleception build Gradle 5.0 changes the behavior of BOM import, which is now required to be imported explicitly using the `platform` keyword. However, this is not yet available on the Gradle version we use to build, so this commit introduces a temporary fix so that we can use the same syntax on all tested versions of Gradle.