Playwright test for device rehydration (#34091)

* Device dehydration tests: use common util for recovery setup

We have a utility method for setting up the recovery key, so let's use it.

* Factor out shared method for creating encrypted rooms

* Playwright test for device dehydration

Add a playwright test to check that this stuff actually works.

apps/web/playwright/e2e/crypto/utils.ts

@@ -406,6 +406,7 @@ export async function copyAndContinue(page: Page) {
  * @param opts - other options for the createRoom call
  *
  * @returns a promise which resolves to the room ID
+ * @see createSharedEncryptedRoomWithUser
  */
 export async function createSharedRoomWithUser(
     app: ElementAppPage,
@@ -423,6 +424,32 @@ export async function createSharedRoomWithUser(
     return roomId;
 }
 
+/**
+ * Create a shared, encrypted room with the given user, and wait for them to join
+ *
+ * @param other - UserID of the other user
+ * @param opts - other options for the createRoom call
+ *
+ * @returns a promise which resolves to the room ID
+ * @see createSharedRoomWithUser
+ */
+export async function createSharedEncryptedRoomWithUser(
+    app: ElementAppPage,
+    other: string,
+    opts: Omit<ICreateRoomOpts, "invite"> = { name: "TestRoom" },
+): Promise<string> {
+    opts = structuredClone(opts);
+    opts.initial_state ??= [];
+    opts.initial_state.push({
+        type: "m.room.encryption",
+        state_key: "",
+        content: {
+            algorithm: "m.megolm.v1.aes-sha2",
+        },
+    });
+    return createSharedRoomWithUser(app, other, opts);
+}
+
 /**
  * Send a message in the current room
  * @param page