test: add a flushPromises

this seems to be needed because `initRustCrypto` now ends up doing slightly
less awaiting

Author: richvdh

spec/unit/rust-crypto/rust-crypto.spec.ts

@@ -77,6 +77,7 @@ import encryptAESSecretStorageItem from "../../../src/utils/encryptAESSecretStor
 import { type CryptoStore, type SecretStorePrivateKeys } from "../../../src/crypto/store/base";
 import { CryptoEvent } from "../../../src/crypto-api/index.ts";
 import { RustBackupManager } from "../../../src/rust-crypto/backup.ts";
+import { flushPromises } from "../../test-utils/flushPromises.ts";
 
 const TEST_USER = "@alice:example.com";
 const TEST_DEVICE_ID = "TEST_DEVICE";
@@ -2302,6 +2303,8 @@ describe("RustCrypto", () => {
             });
 
             const rustCrypto = await makeTestRustCrypto(makeMatrixHttpApi(), undefined, undefined, secretStorage);
+            await flushPromises();
+
             // We have a key backup
             expect(await rustCrypto.getActiveSessionBackupVersion()).not.toBeNull();