Add some roomserver UTs (#3067)

Adds tests for `QueryRestrictedJoinAllowed`, `IsServerAllowed` and
`PerformRoomUpgrade`. Refactors the `QueryRoomVersionForRoom` method to
accept a string and return a `gmsl.RoomVersion` instead of req/resp
structs.
Adds some more caching for `GetStateEvent`

This should also fix #2912 by ignoring state events belonging to other
users.

roomserver/auth/auth.go

@@ -26,6 +26,10 @@ func IsServerAllowed(
 	serverCurrentlyInRoom bool,
 	authEvents []*gomatrixserverlib.Event,
 ) bool {
+	// In practice should not happen, but avoids unneeded CPU cycles
+	if serverName == "" || len(authEvents) == 0 {
+		return false
+	}
 	historyVisibility := HistoryVisibilityForRoom(authEvents)
 
 	// 1. If the history_visibility was set to world_readable, allow.