basic test: Start writing check extensions for basic

Author: sm-sayedi

test/basic_checks.dart

@@ -0,0 +1,6 @@
+import 'package:checks/checks.dart';
+import 'package:zulip/basic.dart';
+
+extension OptionSomeChecks<T> on Subject<OptionSome<T>> {
+  Subject<T> get value => has((x) => x.value, 'value');
+}