add fixture to capture output in tests less fragile

Now we always remove previous output before capturing.

pkgs/clan-cli/tests/stdout.py

@@ -0,0 +1,23 @@
+from typing import Any
+
+import pytest
+from pytest import CaptureFixture
+
+
+class CaptureOutput:
+    def __init__(self, capsys: CaptureFixture) -> None:
+        self.capsys = capsys
+
+    def __enter__(self) -> "CaptureOutput":
+        self.capsys.readouterr()
+        return self
+
+    def __exit__(self, exc_type: Any, exc_value: Any, exc_traceback: Any) -> bool:
+        res = self.capsys.readouterr()
+        self.out = res.out
+        self.err = res.err
+
+
+@pytest.fixture
+def capture_output(capsys: CaptureFixture) -> CaptureOutput:
+    return CaptureOutput(capsys)