aider: Set OPENAI_API_KEY environment variable to "deadbeef" for all tests.

# Context:
USER: set env OPENAI_API_KEY=deadbeef for all tests
ASSISTANT: To set the environment variable `OPENAI_API_KEY` for all tests, you can modify the `setUp` method in the `TestMain` class in the `tests/test_main.py` file. Here's the change:

```python
tests/test_main.py
<<<<<<< ORIGINAL
class TestMain(TestCase):
    def test_main_with_empty_dir_no_files_on_command(self):
=======
class TestMain(TestCase):
    def setUp(self):
        os.environ["OPENAI_API_KEY"] = "deadbeef"

    def test_main_with_empty_dir_no_files_on_command(self):
>>>>>>> UPDATED
```

This will set the `OPENAI_API_KEY` environment variable to "deadbeef" before running each test.

tests/test_main.py

@@ -10,6 +10,9 @@ from prompt_toolkit.output import DummyOutput
 
 
 class TestMain(TestCase):
+    def setUp(self):
+        os.environ["OPENAI_API_KEY"] = "deadbeef"
+
     def test_main_with_empty_dir_no_files_on_command(self):
         with tempfile.TemporaryDirectory() as temp_dir:
             os.chdir(temp_dir)