`TestSendAndFetchMessage` marked as "Flakey" on Dendrite

[ShadowJonathan]

This is an issue pertaining to the following comment and skip;

https://github.com/matrix-org/complement/blob/086fa4bf8f2099198e2f8e6e96cd52da6ce4bbe5/tests/csapi/room_messages_test.go#L21

To see if this can be cleared, and the test made more robust, or a bug in dendrite fixed that causes this flake.