Fix an unrelated non-determinism bug in the system tests

See similar earlier commit

nixos/tests/test_privatestorage.py

@@ -93,6 +93,9 @@ def test(
     # It should have Eliot logging turned on as well.
     storage.succeed('[ -e /var/db/tahoe-lafs/storage/logs/eliot.json ]')
 
+    # Make sure the issuer is ready to accept connections.
+    issuer.wait_for_open_port(80)
+
     #
     # Storage appears to be working so try to get a client to speak with it.
     #