System tests: Start VMs in parallel again

Taken from the `spending-node` branch I am cleaning up
right now.

nixos/tests/test_privatestorage.py

@@ -40,21 +40,8 @@ def test(
 ):
     """
     """
-    # Boot the VMs.  We used to do them all in parallel but the boot
-    # sequence got flaky at some point for some reason I don't
-    # understand. :/ It might be related to this:
-    #
-    # https://discourse.nixos.org/t/nixos-ppc64le-vm-does-not-have-dev-vda-device/11548/9
-    #
-    # See <nixpkgs/nixos/modules/virtualisation/qemu-vm.nix> for the Nix
-    # that constructs the QEMU command that gets run.
-    #
-    # Boot them one at a time for now.
-    issuer.connect()
-    introducer.connect()
-    storage.connect()
-    client.connect()
-    api_stripe_com.connect()
+    # Boot the VMs.
+    start_all()
 
     # The issuer and the storage server should accept SSH connections.  This
     # doesn't prove it is so but if it fails it's a pretty good indication