Add a failing test for issue #985

In order to reproduce the issue, the test creates two `Dataset`s ending
in parallel tasks and sets their `__module__` attribute to a module
below the `egon.data.datasets` package to simulate having the `Dataset`s
created there. Then the test checks that the ids of the final `Dataset`
tasks are distinct.

tests/test_dataset_class.py

@@ -0,0 +1,45 @@
+from dataclasses import dataclass
+from typing import Union
+
+from airflow.models.dag import DAG
+
+from egon.data.datasets import Dataset, TaskGraph, Tasks
+
+
+def test_uniqueness_of_automatically_generated_final_dataset_task():
+    """Test that the generated final dataset task is named uniquely.
+
+    This is a regression test for issue #985. Having multiple `Dataset`s ending
+    in parallel tasks doesn't work if those `Dataset`s are in a module below
+    the `egon.data.datasets` package. In that case the code removing the module
+    name prefix from task ids and the code generating the final dataset task
+    which updates the dataset version once all parallel tasks have finished
+    interact in a way that generates non-distinct task ids so that tasks
+    generated later clobber the ones generated earlier. This leads to spurious
+    cycles and other inconsistencies and bugs in the graph.
+    """
+
+    noops = [(lambda: None) for _ in range(4)]
+    for i, noop in enumerate(noops):
+        noop.__name__ = f"noop-{i}"
+
+    @dataclass
+    class Dataset_1(Dataset):
+        name: str = "DS1"
+        version: str = "0.0.0"
+        tasks: Union[Tasks, TaskGraph] = ({noops[0], noops[1]},)
+
+    @dataclass
+    class Dataset_2(Dataset):
+        name: str = "DS2"
+        version: str = "0.0.0"
+        tasks: Union[Tasks, TaskGraph] = ({noops[2], noops[3]},)
+
+    Dataset_1.__module__ = "egon.data.datasets.test.datasets"
+    Dataset_2.__module__ = "egon.data.datasets.test.datasets"
+    with DAG(dag_id="Test-DAG", default_args={"start_date": "1111-11-11"}):
+        datasets = [Dataset_1(), Dataset_2()]
+    ids = [list(dataset.tasks)[-1] for dataset in datasets]
+    assert (
+        ids[0] != ids[1]
+    ), "Expected unique names for final tasks of distinct datasets."