Test how we handle receiving an unthreaded read receipt

[andybalaam]

As can be seen in the epic #24392, we have found various issues where we didn't understand the difference between unthreaded receipts and receipts in the main thread.

Check that when we receive an unthreaded read receipt we correctly process it.

For example. When the timeline looks like this:

flowchart TD
    A(Unthreaded t=001)
    --> B(Thread root A t=002)
    --> C(Unthreaded t=003)
    --> D(Thread root B t=005)
    --> E(Unthreaded t=007)
    B --> B1(Thread member t=004)
    D --> D1(Thread member t=006) --> D2(Thread member t=008)

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and we receive an unthreaded receipt for Unthreaded t=007, then the main timeline should be read, but thread B should not, because one of its events is after the receipt.