api test: Fix with_ to await callback first, then close

As is, the `finally` block gets run before the future returned by
the callback has completed.  I believe the underlying issue here
is this one in the Dart SDK:
  dart-lang/sdk#44395

The fix is to await the future and return the result of that,
rather than returning the future directly.

The Dart language folks (or at least some of them) hope to
eventually deal with this by making the old code here a type error,
requiring the value passed to `return` to have type T rather
than Future<T>:
  dart-lang/language#870

Author: gnprice

test/api/fake_api.dart

@@ -115,7 +115,7 @@ class FakeApiConnection extends ApiConnection {
       ? FakeApiConnection.fromAccount(account)
       : FakeApiConnection(realmUrl: realmUrl);
     try {
-      return fn(connection);
+      return await fn(connection);
     } finally {
       connection.close();
     }