Test that ForwardDialog can render replies

Previously ForwardDialog was not giving its EventTile message preview
the information it needed to render a ReplyThread. This was a bit tricky
to fix since we were pulling a fake event out of thin air, so this
ensures it doesn't regress.

Signed-off-by: Robin Townsend <robin@robin.town>

test/test-utils.js

@@ -236,6 +236,7 @@ export function mkStubRoom(roomId = null, name) {
         getPendingEvents: () => [],
         getLiveTimeline: () => stubTimeline,
         getUnfilteredTimelineSet: () => null,
+        findEventById: () => null,
         getAccountData: () => null,
         hasMembershipState: () => null,
         getVersion: () => '1',