pkgs/cli: Move the test folder inside the python module

Move the `tests` folder to `clan_cli/tests`.

As we now want part of our tests to live next to the functions that are
tested - tests that are not in the `/tests` module also need access to
the configured test fixtures that are exposed by the `pytest_plugins`
declaration.

The following folder structure doesn't support this model:

```
├── clan_cli
│   ├── api
│   │    └── api_init_test.py
├── tests/
│   ├── conftest.py
│   └── ...
```

Here `api_init_test.py` even when importing the test functions will not
have the fixtures configured.

There is a way to configure python to import the fixtures from another
[`project/module`](https://docs.pytest.org/en/stable/how-to/fixtures.html#using-fixtures-from-other-projects), but this seems to *generally* be discouraged.

So moving the `conftest.py` to the toplevel and the `/tests` folder into
the toplevel seems to be a sensible choice choice.

pkgs/clan-cli/clan_cli/tests/stdout.py

@@ -0,0 +1,34 @@
+import types
+
+import pytest
+
+
+class CaptureOutput:
+    def __init__(self, capsys: pytest.CaptureFixture) -> None:
+        self.capsys = capsys
+        self.capsys_disabled = capsys.disabled()
+        self.capsys_disabled.__enter__()
+
+    def __enter__(self) -> "CaptureOutput":
+        self.capsys_disabled.__exit__(None, None, None)
+        self.capsys.readouterr()
+        return self
+
+    def __exit__(
+        self,
+        exc_type: type[BaseException] | None,
+        exc_value: BaseException | None,
+        traceback: types.TracebackType | None,
+    ) -> None:
+        res = self.capsys.readouterr()
+        self.out = res.out
+        self.err = res.err
+
+        # Disable capsys again
+        self.capsys_disabled = self.capsys.disabled()
+        self.capsys_disabled.__enter__()
+
+
+@pytest.fixture
+def capture_output(capsys: pytest.CaptureFixture) -> CaptureOutput:
+    return CaptureOutput(capsys)