add synchronous test of resubscribe after error

I've added a simple synchronous test of resubscription after error. Retry has had a chequered history of late and I think might benefit from extra tests. The scenario is perhaps covered by the recent additions with #1027 but the test additions for that were async and more complex. This simple scenario broke for versions to 0.16.1 to 0.17.4, hung with 0.17.5 and passes in 0.17.6.