Added async _acall to FakeListLLM (#5439)

# Added Async _acall to FakeListLLM

FakeListLLM is handy when unit testing apps built with langchain. This
allows the use of FakeListLLM inside concurrent code with
[asyncio](https://docs.python.org/3/library/asyncio.html).

I also changed the pydocstring which was out of date.

## Who can review?

@hwchase17 - project lead
@agola11 - async

langchain/llms/fake.py

@@ -1,7 +1,10 @@
 """Fake LLM wrapper for testing purposes."""
 from typing import Any, List, Mapping, Optional
 
-from langchain.callbacks.manager import CallbackManagerForLLMRun
+from langchain.callbacks.manager import (
+    AsyncCallbackManagerForLLMRun,
+    CallbackManagerForLLMRun,
+)
 from langchain.llms.base import LLM
 
 
@@ -22,7 +25,18 @@ class FakeListLLM(LLM):
         stop: Optional[List[str]] = None,
         run_manager: Optional[CallbackManagerForLLMRun] = None,
     ) -> str:
-        """First try to lookup in queries, else return 'foo' or 'bar'."""
+        """Return next response"""
+        response = self.responses[self.i]
+        self.i += 1
+        return response
+
+    async def _acall(
+        self,
+        prompt: str,
+        stop: Optional[List[str]] = None,
+        run_manager: Optional[AsyncCallbackManagerForLLMRun] = None,
+    ) -> str:
+        """Return next response"""
         response = self.responses[self.i]
         self.i += 1
         return response