Make the test pass by waiting a little longer.

Also, comments.

nixos/modules/tests/private-storage.nix

@@ -13,8 +13,19 @@ import <nixpkgs/nixos/tests/make-test.nix> {
   # Test the machine with a Perl program (sobbing).
   testScript =
     ''
+      # Boot the VM.
       $machine->start;
+
+      # The systemd unit should reach the running state.
       $machine->waitForUnit("tahoe.storage.service");
+
+      # Some while after that the Tahoe-LAFS node should listen on the web API
+      # port. The port number here has to agree with the port number set in
+      # the private-storage.nix module.
+      $machine->waitForOpenPort(3456);
+
+      # Once the web API is listening it should be possible to scrape some
+      # status from the node if it is really working.
       $machine->succeed("tahoe -d /var/db/tahoe-lafs/storage status");
     '';
 }