tests: Fix conftest.

This change fixes tests with Pytest collection:

```
Using pytest
ImportError while loading conftest '/tmp/guix-build-python-asyncclick-8.2.2.2.drv-0/asyncclick-8.2.2.2/tests/conftest.py'.
tests/conftest.py:3: in <module>
    from click.testing import CliRunner
E   ModuleNotFoundError: No module named 'click'
```
Ref:

Author: Hellseher

tests/conftest.py

@@ -1,6 +1,6 @@
 import pytest
 
-from click.testing import CliRunner
+from asyncclick.testing import CliRunner
 
 
 @pytest.fixture(scope="function")