test: Ensure that all databases are always closed on exit (#1187)

This is really important for file based datastores, and failure to close even a couple will have a highly significant performance impact due to a long-term defra bug, where if the directory hosting defra is deleted whilst a database instance is active the instance will hang/infinate-loop chewing up a huge amount of resources. As the file based directories are deleted by `go test` at the end of each test, this magnifies the effect of any lingering instances.  In the case of this issue I believe only a very small number of databases were leaked, yet it nearly doubled the execution time of the change detector (the only potential leaks I found were a couple of panics, and a couple of subscription tests, it could be that as few as 3-4 leaked databases were responsible for the loss of performance).

tests/integration/utils2.go

@@ -307,6 +307,10 @@ func executeTestCase(
 	syncChans := []chan struct{}{}
 	nodeAddresses := []string{}
 	nodes := getStartingNodes(ctx, t, dbt, collectionNames, testCase)
+	// It is very important that the databases are always closed, otherwise resources will leak
+	// as tests run.  This is particularly important for file based datastores.
+	defer closeNodes(ctx, t, &nodes)
+
 	// Documents and Collections may already exist in the database if actions have been split
 	// by the change detector so we should fetch them here at the start too (if they exist).
 	// collections are by node (index), as they are specific to nodes.
@@ -400,8 +404,15 @@ func executeTestCase(
 			assert.Fail(t, "timeout occurred while waiting for data stream", testCase.Description)
 		}
 	}
+}
 
-	for _, node := range nodes {
+// closeNodes closes all the given nodes, ensuring that resources are properly released.
+func closeNodes(
+	ctx context.Context,
+	t *testing.T,
+	nodes *[]*node.Node,
+) {
+	for _, node := range *nodes {
 		if node.Peer != nil {
 			err := node.Close()
 			require.NoError(t, err)