test [nfc]: Add package:checks getters for ApiConnection

Author: gnprice

test/api/core_checks.dart

@@ -0,0 +1,7 @@
+import 'package:checks/checks.dart';
+import 'package:zulip/api/core.dart';
+
+extension ApiConnectionChecks on Subject<ApiConnection> {
+  Subject<Uri> get realmUrl => has((x) => x.realmUrl, 'realmUrl');
+  Subject<int?> get zulipFeatureLevel => has((x) => x.zulipFeatureLevel, 'zulipFeatureLevel');
+}