aider: Added tests for key functionality of udiff coder.

# Aider chat conversation:

USER: write some basic tests for the key functionality of udiff coder

ASSISTANT: ```diff
--- tests/test_udiff.py
+++ tests/test_udiff.py
@@ ... @@
+import unittest
+from unittest.mock import MagicMock, patch
+from pathlib import Path
+from aider.coders.udiff_coder import UnifiedDiffCoder, do_replace, apply_hunk, find_diffs
+from aider.io import InputOutput
+
+class TestUnifiedDiffCoder(unittest.TestCase):
+    def setUp(self):
+        self.coder = UnifiedDiffCoder(io=InputOutput())
+
+    def test_do_replace_new_file(self):
+        # Test do_replace when it should create a new file
+        hunk = [
+            "--- /dev/null",
+            "+++ newfile.txt",
+            "@@ -0,0 +1 @@",
+            "+New content\n"
+        ]
+        result = do_replace('newfile.txt', None, hunk)
+        self.assertEqual(result, 'New content\n')
+
+    def test_apply_hunk_with_changes(self):
+        # Test apply_hunk with actual changes
+        content = "Original line 1\nOriginal line 2\n"
+        hunk = [
+            "@@ -1,2 +1,2 @@",
+            " Original line 1",
+            "-Original line 2",
+            "+Modified line 2"
+        ]
+        result = apply_hunk(content, hunk)
+        self.assertEqual(result, "Original line 1\nModified line 2\n")
+
+    def test_find_diffs_single_hunk(self):
+        # Test find_diffs with a single hunk
+        content = "```diff\n--- a/file.txt\n+++ b/file.txt\n@@ -1,2 +1,2 @@\n-Original\n+Modified\n```\n"
+        result = find_diffs(content)
+        self.assertEqual(len(result), 1)
+        self.assertEqual(result[0][0], 'b/file.txt')
+        self.assertEqual(result[0][1], ['@@ -1,2 +1,2 @@\n', '-Original\n', '+Modified\n'])
+
+if __name__ == '__main__':
+    unittest.main()
```

tests/test_udiff.py

@@ -0,0 +1,43 @@
+import unittest
+from unittest.mock import MagicMock, patch
+from pathlib import Path
+from aider.coders.udiff_coder import UnifiedDiffCoder, do_replace, apply_hunk, find_diffs
+from aider.io import InputOutput
+
+class TestUnifiedDiffCoder(unittest.TestCase):
+    def setUp(self):
+        self.coder = UnifiedDiffCoder(io=InputOutput())
+
+    def test_do_replace_new_file(self):
+        # Test do_replace when it should create a new file
+        hunk = [
+            "--- /dev/null",
+            "+++ newfile.txt",
+            "@@ -0,0 +1 @@",
+            "+New content\n"
+        ]
+        result = do_replace('newfile.txt', None, hunk)
+        self.assertEqual(result, 'New content\n')
+
+    def test_apply_hunk_with_changes(self):
+        # Test apply_hunk with actual changes
+        content = "Original line 1\nOriginal line 2\n"
+        hunk = [
+            "@@ -1,2 +1,2 @@",
+            " Original line 1",
+            "-Original line 2",
+            "+Modified line 2"
+        ]
+        result = apply_hunk(content, hunk)
+        self.assertEqual(result, "Original line 1\nModified line 2\n")
+
+    def test_find_diffs_single_hunk(self):
+        # Test find_diffs with a single hunk
+        content = "```diff\n--- a/file.txt\n+++ b/file.txt\n@@ -1,2 +1,2 @@\n-Original\n+Modified\n```\n"
+        result = find_diffs(content)
+        self.assertEqual(len(result), 1)
+        self.assertEqual(result[0][0], 'b/file.txt')
+        self.assertEqual(result[0][1], ['@@ -1,2 +1,2 @@\n', '-Original\n', '+Modified\n'])
+
+if __name__ == '__main__':
+    unittest.main()