store test: Add await for checking Future.throws.

These tests actually worked as expected, even though we didn't await
for the Future returned by FutureChecks.throws.

This is because checks ensure that the asychronous expectations are
complete before the test ends.

Nonetheless, we should use await so that our intention is explicit
(i.e. the future completes with an error as opposed to the future
never completes and can't be awaited).

See also:
  https://github.com/dart-lang/test/tree/master/pkgs/checks#checking-asynchronous-expectations

Signed-off-by: Zixuan James Li <zixuan@zulip.com>

Author: PIG208

test/model/store_test.dart

@@ -153,11 +153,11 @@ void main() {
 
     test('reject changing id, realmUrl, or userId', () async {
       final globalStore = eg.globalStore(accounts: [eg.selfAccount]);
-      check(globalStore.updateAccount(eg.selfAccount.id, const AccountsCompanion(
+      await check(globalStore.updateAccount(eg.selfAccount.id, const AccountsCompanion(
         id: Value(1234)))).throws();
-      check(globalStore.updateAccount(eg.selfAccount.id, AccountsCompanion(
+      await check(globalStore.updateAccount(eg.selfAccount.id, AccountsCompanion(
         realmUrl: Value(Uri.parse('https://other.example'))))).throws();
-      check(globalStore.updateAccount(eg.selfAccount.id, const AccountsCompanion(
+      await check(globalStore.updateAccount(eg.selfAccount.id, const AccountsCompanion(
         userId: Value(1234)))).throws();
     });