Add unit tests for the latest changes to merge

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

tests/test_merge.py

@@ -0,0 +1,24 @@
+# License: MIT
+# Copyright © 2022 Frequenz Energy-as-a-Service GmbH
+
+"""Tests for the merge implementation."""
+
+from unittest import mock
+
+import pytest
+
+from frequenz.channels import Receiver, merge
+
+
+async def test_empty() -> None:
+    """Ensure merge() raises an exception when no receivers are provided."""
+    with pytest.raises(ValueError, match="At least one receiver must be provided"):
+        merge()
+
+
+async def test_one() -> None:
+    """Ensure merge() returns the same receiver when only one is provided."""
+    receiver = mock.MagicMock(spec=Receiver[int])
+
+    merge_receiver: Receiver[int] = merge(receiver)
+    assert merge_receiver is receiver