Fix limit logic for EventsStream (#7358)

* Factor out functions for injecting events into database

I want to add some more flexibility to the tools for injecting events into the
database, and I don't want to clutter up HomeserverTestCase with them, so let's
factor them out to a new file.

* Rework TestReplicationDataHandler

This wasn't very easy to work with: the mock wrapping was largely superfluous,
and it's useful to be able to inspect the received rows, and clear out the
received list.

* Fix AssertionErrors being thrown by EventsStream

Part of the problem was that there was an off-by-one error in the assertion,
but also the limit logic was too simple. Fix it all up and add some tests.

tests/test_utils/__init__.py

@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 # Copyright 2019 New Vector Ltd
+# Copyright 2020 The Matrix.org Foundation C.I.C
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -16,3 +17,22 @@
 """
 Utilities for running the unit tests
 """
+from typing import Awaitable, TypeVar
+
+TV = TypeVar("TV")
+
+
+def get_awaitable_result(awaitable: Awaitable[TV]) -> TV:
+    """Get the result from an Awaitable which should have completed
+
+    Asserts that the given awaitable has a result ready, and returns its value
+    """
+    i = awaitable.__await__()
+    try:
+        next(i)
+    except StopIteration as e:
+        # awaitable returned a result
+        return e.value
+
+    # if next didn't raise, the awaitable hasn't completed.
+    raise Exception("awaitable has not yet completed")