aider: Added a test for cmd_add that passes in 'foo.txt' and 'bar.txt' and ensures they have both been created after the test.

# Context:
USER: add a test for cmd_add that passes in 'foo.txt' and 'bar.txt' and ensures they have both been created after the test. run the test in a tmp dir.
ASSISTANT: To add the test for `cmd_add`, we will modify the `tests/test_commands.py` file. Here's the step-by-step plan:

1. Import the necessary libraries and classes.
2. Create a new test method called `test_cmd_add` inside the `TestCommands` class.
3. In the `test_cmd_add` method, create a temporary directory using `tempfile.TemporaryDirectory`.
4. Change the current working directory to the temporary directory.
5. Create an instance of the `InputOutput` and `Coder` classes.
6. Create an instance of the `Commands` class, passing the `InputOutput` and `Coder` instances.
7. Call the `cmd_add` method with the arguments `['foo.txt', 'bar.txt']`.
8. Check if both 'foo.txt' and 'bar.txt' files have been created in the temporary directory.
9. Change the current working directory back to the original directory.

Here's the code change:

```python
tests/test_commands.py
<<<<<<< ORIGINAL
=======
import os
import tempfile
from unittest import TestCase
from aider.io import InputOutput
from aider.coder import Coder
from aider.commands import Commands

class TestCommands(TestCase):
    def test_cmd_add(self):
        with tempfile.TemporaryDirectory() as tmpdir:
            original_cwd = os.getcwd()
            os.chdir(tmpdir)

            io = InputOutput()
            coder = Coder(io)
            commands = Commands(io, coder)

            commands.cmd_add(['foo.txt', 'bar.txt'])

            self.assertTrue(os.path.exists('foo.txt'))
            self.assertTrue(os.path.exists('bar.txt'))

            os.chdir(original_cwd)
>>>>>>> UPDATED
```

This will add the test for `cmd_add` and ensure that both 'foo.txt' and 'bar.txt' have been created after the test.

tests/test_commands.py

@@ -0,0 +1,23 @@
+import os
+import tempfile
+from unittest import TestCase
+from aider.io import InputOutput
+from aider.coder import Coder
+from aider.commands import Commands
+
+class TestCommands(TestCase):
+    def test_cmd_add(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            original_cwd = os.getcwd()
+            os.chdir(tmpdir)
+
+            io = InputOutput()
+            coder = Coder(io)
+            commands = Commands(io, coder)
+
+            commands.cmd_add(['foo.txt', 'bar.txt'])
+
+            self.assertTrue(os.path.exists('foo.txt'))
+            self.assertTrue(os.path.exists('bar.txt'))
+
+            os.chdir(original_cwd)