Add subscription drop/resubscribe tests

[alexeyzimarev]

For all the available stores (relational and ESDB) add tests to ensure that the subscription drop causes resubscribe.

Also ensure that the health check returns degraded when the subscription is dropped.

I've seen that resubscribe logic works fine for ESDB as when I run the sample app and stop the ESDB container, it enters the loop of doom trying to resubscribe. As soon as the container starts again, the subscription restores. For SQL Server, #307 indicates that it doesn't resubscribe, so it's best to start with adding a test there.