It isn't writing the test case that's the problem - the same technique we're using for the synchronous CM works for the asynchronous CM as well.
The problem is that unlike the synchronous CM, the DEFER_PENDING_UNTIL opcode isn't sufficient to make the async CM test case *pass*, as "async with" yields control back to the event loop at the YIELD_FROM points, and hence it's always going to be possible for signals to interrupt the transfer of control to and from __aenter__ or __aexit__, even if those methods are themselves implemented in C.