private UnionFileCollection newUnionFileCollection(FileCollectionInternal... source) {
        new UnionFileCollection(TestFiles.taskDependencyFactory(), source)
    }

    def containsUnionOfAllSourceCollections() {
        def source1 = new TestFileCollection(file1, file2)
        def source2 = new TestFileCollection(file2, file3)

        expect:
        def collection = newUnionFileCollection(source1, source2)