Tests sometime log datastore close error but still pass

[AndrewSisley]

It looks like something is still occasionally trying to access the datastore after the database has been closed. The test still passed in this case, but there is a chance that it may be occasionally responsible for test flakiness. Rate of occurrence seems quite low (<~= 1:100), but I have seen it a few times now.

gotestsum --format testname -- ./tests/integration/subscription -race -shuffle=on
PASS tests/integration/subscription.TestSubscriptionWithUpdateAllMutations (0.21s)
PASS tests/integration/subscription.TestSubscriptionWithFilterAndOneCreateMutation (0.16s)
PASS tests/integration/subscription.TestSubscriptionWithFilterAndCreateMutations (0.12s)
PASS tests/integration/subscription.TestSubscriptionWithCreateMutations (0.13s)
PASS tests/integration/subscription.TestSubscriptionWithUpdateMutations (0.12s)
PASS tests/integration/subscription.TestSubscriptionWithFilterAndOneCreateMutationOutsideFilter (0.11s)
2023-04-13T16:36:42.440Z, ERROR, defra.db, datastore closed
PASS tests/integration/subscription

My best guess so far is that this is being logged in db/subscriptions.go handleSubscription, with an event being received after close:

for evt := range pub.Event() {
		txn, err := db.NewTxn(ctx, false)
		if err != nil {
			log.Error(ctx, err.Error())
			continue
		}

*simpleChannel[T].Close() is asynchronous, and does leave room for it to not have fully closed before the root datastore has closed. It would however be odd that an 'extra' event could be received after the datastore has been closed - this would suggest that the test is exiting prematurely, and that extra, unexpected, events are being received.

[AndrewSisley]

Related to: #1351