This patch renames tests.

This patch implements and tests
`SlidingSyncListLoadingState::is_fully_loaded` for more convenience.

This patch adds a small helper:
`SlidingSyncListRequestGenerator::is_selective`.

This patchs adds the `SlidingSyncList::requires_timeout` method to know
exactly when a list should trigger a `timeout` on the request.

This patch updates when sliding sync requests have a `timeout`.

Prior to this patch, all requests had a `timeout` query, set to the
`poll_timeout` duration value. However it means: if there is no data
to return, wait `timeout` milliseconds for new data before returning.
This definition is correct. Problem: if the current range of a list
has no data, the server will wait! It means that, in a situation where
there is no update at all, but the client is fetching all rooms batch by
batch, it will wait `poll_timeout` for each batch!

The behaviour described above is absolutely correct. Some server
implementations are less strict though, and we didn't realise our code
was doing that, because the server had some optimisations to ignore the
timeout if the range wasn't covering all the rooms. Nonetheless, a new
server implementation (namely Synapse) is strict, and it confirms we
have a bug here.

This patch then configures a `timeout` if all lists require a timeout,
otherwise there is no `timeout`, which is equivalent to `timeout=0`.

This patch is a small cleanup.