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.
