Add unit test for _merge_splits function (#3513)

This commit adds a new unit test for the _merge_splits function in the
text splitter. The new test verifies that the function merges text into
chunks of the correct size and overlap, using a specified separator. The
test passes on the current implementation of the function.

tests/unit_tests/test_text_splitter.py

@@ -68,6 +68,15 @@ def test_character_text_splitting_args() -> None:
         CharacterTextSplitter(chunk_size=2, chunk_overlap=4)
 
 
+def test_merge_splits() -> None:
+    """Test merging splits with a given separator."""
+    splitter = CharacterTextSplitter(separator=" ", chunk_size=9, chunk_overlap=2)
+    splits = ["foo", "bar", "baz"]
+    expected_output = ["foo bar", "baz"]
+    output = splitter._merge_splits(splits, separator=" ")
+    assert output == expected_output
+
+
 def test_create_documents() -> None:
     """Test create documents method."""
     texts = ["foo bar", "baz"]