Faster room joins: Add happy path tests for outgoing device list updates

[squahtx]

This change tests that:

• device list updates are sent to the servers listed in the
  /send_join response
• device list updates are sent to servers that join after we start the
  partial state join
• device list updates stop being sent to servers listed in the
  /send_join response that are no longer in the room once the partial
  state join completes.

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