add missing RemoteLangChainRetriever _get_relevant_documents test (#8628)

# What
- Add missing RemoteLangChainRetriever _get_relevant_documents test

---------

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

libs/langchain/tests/unit_tests/retrievers/test_remote_retriever.py

@@ -0,0 +1,60 @@
+from typing import Any, Dict
+
+from pytest_mock import MockerFixture
+
+from langchain.retrievers import RemoteLangChainRetriever
+from langchain.schema import Document
+
+
+class MockResponse:
+    def __init__(self, json_data: Dict, status_code: int):
+        self.json_data = json_data
+        self.status_code = status_code
+
+    def json(self) -> Dict:
+        return self.json_data
+
+
+def mocked_requests_post(*args: Any, **kwargs: Any) -> MockResponse:
+    return MockResponse(
+        json_data={
+            "response": [
+                {
+                    "page_content": "I like apples",
+                    "metadata": {
+                        "test": 0,
+                    },
+                },
+                {
+                    "page_content": "I like pineapples",
+                    "metadata": {
+                        "test": 1,
+                    },
+                },
+            ]
+        },
+        status_code=200,
+    )
+
+
+def test_RemoteLangChainRetriever_get_relevant_documents(
+    mocker: MockerFixture,
+) -> None:
+    mocker.patch("requests.post", side_effect=mocked_requests_post)
+
+    remote_langchain_retriever = RemoteLangChainRetriever(
+        url="http://localhost:8000",
+    )
+    response = remote_langchain_retriever.get_relevant_documents("I like apples")
+    want = [
+        Document(page_content="I like apples", metadata={"test": 0}),
+        Document(page_content="I like pineapples", metadata={"test": 1}),
+    ]
+
+    assert len(response) == len(want)
+    for r, w in zip(response, want):
+        assert r.page_content == w.page_content
+        assert r.metadata == w.metadata
+
+
+# TODO: _aget_relevant_documents test