test: EventBuilder<RoomCreateEventContent> can set predecessor.

This patch adds the `predecessor` and `no_predecessor` methods on
`EventBuilder<RoomCreateEventContent>`. This is helpful to configure the
`predecessor` field.

testing/matrix-sdk-test/src/event_factory.rs

@@ -41,7 +41,7 @@ use ruma::{
         room::{
             avatar::{self, RoomAvatarEventContent},
             canonical_alias::RoomCanonicalAliasEventContent,
-            create::RoomCreateEventContent,
+            create::{PreviousRoom, RoomCreateEventContent},
             encrypted::{EncryptedEventScheme, RoomEncryptedEventContent},
             member::{MembershipState, RoomMemberEventContent},
             message::{
@@ -400,6 +400,20 @@ impl EventBuilder<UnstablePollStartEventContent> {
     }
 }
 
+impl EventBuilder<RoomCreateEventContent> {
+    /// Define the predecessor fields.
+    pub fn predecessor(mut self, room_id: &RoomId, event_id: &EventId) -> Self {
+        self.content.predecessor = Some(PreviousRoom::new(room_id.to_owned(), event_id.to_owned()));
+        self
+    }
+
+    /// Erase the predecessor if any.
+    pub fn no_predecessor(mut self) -> Self {
+        self.content.predecessor = None;
+        self
+    }
+}
+
 impl<E: EventContent> From<EventBuilder<E>> for Raw<AnySyncTimelineEvent>
 where
     E::EventType: Serialize,