aider: Updated test to pass a string instead of a list when calling cmd_add method.

# Context:
USER: I ran this command:

 pytest tests/test_commands.py

Which produced this output:

============================= test session starts ==============================
platform darwin -- Python 3.11.3, pytest-7.3.1, pluggy-1.0.0
rootdir: /Users/gauthier/Projects/aider
collected 1 item

tests/test_commands.py F                                                 [100%]

=================================== FAILURES ===================================
__________________________ TestCommands.test_cmd_add ___________________________

self = <tests.test_commands.TestCommands testMethod=test_cmd_add>

    def test_cmd_add(self):
        # Create a temporary directory and change the current working directory
        with tempfile.TemporaryDirectory() as tmpdir:
            os.chdir(tmpdir)

            # Initialize the Commands and InputOutput objects
            io = InputOutput(pretty=False, yes=True)
            coder = MagicMock()
            commands = Commands(io, coder)

            # Call the cmd_add method with 'foo.txt' and 'bar.txt'
>           commands.cmd_add(["foo.txt", "bar.txt"])

/Users/gauthier/Projects/aider/tests/test_commands.py:20:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <aider.commands.Commands object at 0x103e2fa50>
args = ['foo.txt', 'bar.txt']

    def cmd_add(self, args):
        "Add matching files to the chat session"

        added_fnames = []
        files = self.coder.get_all_relative_files()
>       for word in args.split():
E       AttributeError: 'list' object has no attribute 'split'

/Users/gauthier/Projects/aider/aider/commands.py:161: AttributeError
=========================== short test summary info ============================
FAILED tests/test_commands.py::TestCommands::test_cmd_add - AttributeError: '...
============================== 1 failed in 0.28s ===============================

ASSISTANT: It seems that the `cmd_add` method in `aider/commands.py` is expecting a string as input, but in the test, we are passing a list. I will update the test to pass a string instead of a list.

Here's the code change:

```python
tests/test_commands.py
<<<<<<< ORIGINAL
            # Call the cmd_add method with 'foo.txt' and 'bar.txt'
            commands.cmd_add(["foo.txt", "bar.txt"])
=======
            # Call the cmd_add method with 'foo.txt' and 'bar.txt' as a single string
            commands.cmd_add("foo.txt bar.txt")
>>>>>>> UPDATED
```

Now, run the test again with the updated test method:

```
pytest tests/test_commands.py
```

tests/test_commands.py

@@ -16,8 +16,8 @@ class TestCommands(TestCase):
             coder = MagicMock()
             commands = Commands(io, coder)
 
-            # Call the cmd_add method with 'foo.txt' and 'bar.txt'
-            commands.cmd_add(["foo.txt", "bar.txt"])
+            # Call the cmd_add method with 'foo.txt' and 'bar.txt' as a single string
+            commands.cmd_add("foo.txt bar.txt")
 
             # Check if both files have been created in the temporary directory
             self.assertTrue(os.path.exists("foo.txt"))