aider: Added assertion to confirm that foo.txt exists after main exits in test_main_with_empty_dir_new_file.

# Context:
USER: in test_main_with_empty_dir_new_file, confirm that foo.txt exists after main exists
ASSISTANT: To confirm that `foo.txt` exists after `main` exits, you can add an assertion in the `test_main_with_empty_dir_new_file` test case. Here's the change:

```python
tests/test_main.py
<<<<<<< ORIGINAL
                main(['foo.txt'])
                sys.stdin = save_stdin
=======
                main(['foo.txt'])
                sys.stdin = save_stdin
                self.assertTrue(os.path.exists('foo.txt'))
>>>>>>> UPDATED
```

tests/test_main.py

@@ -22,3 +22,4 @@ class TestMain(TestCase):
                 sys.stdin = dev_null
                 main(['foo.txt'])
                 sys.stdin = save_stdin
+                self.assertTrue(os.path.exists('foo.txt'))