Move editing test into thread.spec.ts (#3408)
* 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
