Simplify thread-editing test (#3409)

* Fix an existing test for editing messages in threads

While attempting to test a new change, I discovered that the test
"should allow edits to be added to thread timeline" did not actually
fail if its assertions failed. Further, those assertions were incorrect.

So this change fixes the test to create the thread, wait for it to be
initialised, and then add events to it. This simplifies the flow and
ensures the test fails if something unexpected happens.

* Move editing test into thread.spec.ts

* Isolate Thread global modification in beforeAll()

* Delete unneeded setUnsigned call

* Use standard message-creation methods

* Rename event variables

* Rename sender->user

* Remove unneeded variables

* Extract distractions into functions

spec/test-utils/test-utils.ts

@@ -370,6 +370,37 @@ export function mkReaction(
     );
 }
 
+export function mkEdit(
+    target: MatrixEvent,
+    client: MatrixClient,
+    userId: string,
+    roomId: string,
+    msg?: string,
+    ts?: number,
+) {
+    msg = msg ?? `Edit of ${target.getId()}`;
+    return mkEvent(
+        {
+            event: true,
+            type: EventType.RoomMessage,
+            user: userId,
+            room: roomId,
+            content: {
+                "body": `* ${msg}`,
+                "m.new_content": {
+                    body: msg,
+                },
+                "m.relates_to": {
+                    rel_type: RelationType.Replace,
+                    event_id: target.getId()!,
+                },
+            },
+            ts,
+        },
+        client,
+    );
+}
+
 /**
  * A mock implementation of webstorage
  */