Avoid leaving any file after running tests

This notably prevents the file `.nx_file` from being created and not
removed after running tests.  That file could also lead to confusing
test failures when changing and testing the code of python-dotenv.

Author: bbc2

tests/conftest.py

@@ -4,7 +4,9 @@
 
 @pytest.fixture
 def cli():
-    yield CliRunner()
+    runner = CliRunner()
+    with runner.isolated_filesystem():
+        yield runner
 
 
 @pytest.fixture