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/replication/tcp/streams/test_receipts.py

@@ -12,6 +12,11 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+
+# type: ignore
+
+from mock import Mock
+
 from synapse.replication.tcp.streams._base import ReceiptsStream
 
 from tests.replication.tcp.streams._base import BaseStreamTestCase
@@ -20,11 +25,14 @@ USER_ID = "@feeling:blue"
 
 
 class ReceiptsStreamTestCase(BaseStreamTestCase):
+    def _build_replication_data_handler(self):
+        return Mock(wraps=super()._build_replication_data_handler())
+
     def test_receipt(self):
         self.reconnect()
 
         # make the client subscribe to the receipts stream
-        self.test_handler.streams.add("receipts")
+        self.test_handler.stream_positions.update({"receipts": 0})
 
         # tell the master to send a new receipt
         self.get_success(