add test for knn retriever (#7769)

# What
- This is to add test for knn retriever.
---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

tests/unit_tests/retrievers/test_knn.py

@@ -0,0 +1,11 @@
+from langchain.embeddings import FakeEmbeddings
+from langchain.retrievers.knn import KNNRetriever
+
+
+class TestKNNRetriever:
+    def test_from_texts(self) -> None:
+        input_texts = ["I have a pen.", "Do you have a pen?", "I have a bag."]
+        knn_retriever = KNNRetriever.from_texts(
+            texts=input_texts, embeddings=FakeEmbeddings(size=100)
+        )
+        assert len(knn_retriever.texts) == 3