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/command.py

@@ -0,0 +1,66 @@
+import contextlib
+import os
+import signal
+import subprocess
+from collections.abc import Iterator
+from pathlib import Path
+from typing import IO, Any
+
+import pytest
+
+_FILE = None | int | IO[Any]
+
+
+class Command:
+    def __init__(self) -> None:
+        self.processes: list[subprocess.Popen[str]] = []
+
+    def run(
+        self,
+        command: list[str],
+        extra_env: dict[str, str] | None = None,
+        stdin: _FILE = None,
+        stdout: _FILE = None,
+        stderr: _FILE = None,
+        workdir: Path | None = None,
+    ) -> subprocess.Popen[str]:
+        if extra_env is None:
+            extra_env = {}
+        env = os.environ.copy()
+        env.update(extra_env)
+        # We start a new session here so that we can than more reliably kill all children as well
+        p = subprocess.Popen(
+            command,
+            env=env,
+            start_new_session=True,
+            stdout=stdout,
+            stderr=stderr,
+            stdin=stdin,
+            text=True,
+            cwd=workdir,
+        )
+        self.processes.append(p)
+        return p
+
+    def terminate(self) -> None:
+        # Stop in reverse order in case there are dependencies.
+        # We just kill all processes as quickly as possible because we don't
+        # care about corrupted state and want to make tests fasts.
+        for p in reversed(self.processes):
+            with contextlib.suppress(OSError):
+                os.killpg(os.getpgid(p.pid), signal.SIGKILL)
+                p.wait()
+
+
+@pytest.fixture
+def command() -> Iterator[Command]:
+    """
+    Starts a background command. The process is automatically terminated in the end.
+    >>> p = command.run(["some", "daemon"])
+    >>> print(p.pid)
+    """
+    c = Command()
+    try:
+        yield c
+    finally:
+        c.terminate()