[objective_c] Memory leak in ns_input_stream_test.dart

[liamappelbe]

There's a memory leak in ns_input_stream_test.dart, but it's not reproducible in real apps. @brianquinlan has verified that DartInputStreamAdapter is correctly cleaned up in a flutter app. I've verified that the inputStream variable in both tests is being GC'd. So there's just an extra retain floating around in the test infrastructure that we haven't been able to track down.