Pass lazy loading filter to sync reqs in room summary tests

[Fizzadar]

This aligns correctly with the spec and is also seemingly the only reason the test is disabled on synapse, now re-enabled.

[squahtx]

lgtm

[S7evinK]

Also see #553 and matrix-org/matrix-spec#1325, where we came to the conclusion, for now, that it's not clear by the spec when this field should be added.
Looking at RoomSummary and the m.heroes, it states that this is required, if the room has neither m.room.name nor m.room.canonical_alias. So given a room can have neither (e.g. DMs), shouldn't Synapse include a RoomSummary, no matter if lazy loading or not?

[squahtx]

Also see #553 and matrix-org/matrix-spec#1325, where we came to the conclusion, for now, that it's not clear by the spec when this field should be added. Looking at RoomSummary and the m.heroes, it states that this is required, if the room has neither m.room.name nor m.room.canonical_alias. So given a room can have neither (e.g. DMs), shouldn't Synapse include a RoomSummary, no matter if lazy loading or not?

The spec's a little unclear, since it doesn't directly mention when summary is required. However it makes sense for the server to send m.heroes for DMs, for example. If we take summary to be required when any of its fields are required, then Synapse is not spec compliant here.

I've filed this as matrix-org/synapse#15017.