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.
Add some test coverage to verify that each of the IDE plugins can be applied to `buildSrc`. Start growing some cross-cutting test coverage for the IDE plugins, so the plugins can become less different to each other.