test [nfc]: Make addSubscriptions async

This is the next step toward making handleEvent async.

Author: gnprice

test/model/test_store.dart

@@ -133,10 +133,10 @@ extension PerAccountStoreTestExtension on PerAccountStore {
   }
 
   Future<void> addSubscription(Subscription subscription) async {
-    addSubscriptions([subscription]);
+    await addSubscriptions([subscription]);
   }
 
-  void addSubscriptions(List<Subscription> subscriptions) {
+  Future<void> addSubscriptions(List<Subscription> subscriptions) async {
     handleEvent(SubscriptionAddEvent(id: 1, subscriptions: subscriptions));
   }
 

test/widgets/inbox_test.dart

@@ -62,7 +62,7 @@ void main() {
     store = await testBinding.globalStore.perAccount(eg.selfAccount.id);
 
     await store.addStreams(streams ?? []);
-    store.addSubscriptions(subscriptions ?? []);
+    await store.addSubscriptions(subscriptions ?? []);
     await store.addUsers(users ?? [eg.selfUser]);
 
     for (final message in unreadMessages) {