add basic tests for returncode-attribute

This commit adds some basic tests to verify
that the `returncode` attribute of the
`iterable_subprocess` generator is properly
set.

datalad_next/iterable_subprocess/test_iterable_subprocess.py

@@ -312,3 +312,26 @@ def yield_input():
 
     with iterable_subprocess(['funzip'], yield_input()) as output:
         assert b''.join(output) == contents
+
+
+def test_error_returncode_available_from_generator():
+    with pytest.raises(IterableSubprocessError):
+        with iterable_subprocess(['ls', 'does-not-exist'], ()) as ls:
+            tuple(ls)
+    assert ls.returncode != 0
+
+
+def test_error_returncode_available_from_generator_with_exception():
+    with pytest.raises(StopIteration):
+        with iterable_subprocess(['ls', 'does-not-exist'], ()) as ls:
+            while True:
+                next(ls)
+    assert ls.returncode != 0
+
+
+def test_success_returncode_available_from_generator_with_exception():
+    with pytest.raises(StopIteration):
+        with iterable_subprocess(['echo', 'a'], ()) as echo:
+            while True:
+                next(echo)
+    assert echo.returncode == 0