tests/nixos/remote-builds: Wait for multi-user

This should make the test more robust, considering the strange hang
in https://hydra.nixos.org/build/267517233/nixlog/8

`builder` seems to have reached `multi-user.target` before the
SSH connection was established, but this seems to be coincidental.
This does tell us that enforcing this has a minimal cost in terms
of runtime.

Waiting for `multi-user.target` on the client is honestly paranoid,
but flaky tests are very bad for productivity.

tests/nixos/remote-builds.nix

@@ -104,6 +104,11 @@ in
         builder.succeed("mkdir -p -m 700 /root/.ssh")
         builder.copy_from_host("key.pub", "/root/.ssh/authorized_keys")
         builder.wait_for_unit("sshd")
+        # Make sure the builder can handle our login correctly
+        builder.wait_for_unit("multi-user.target")
+        # Make sure there's no funny business on the client either
+        # (should not be necessary, but we have reason to be careful)
+        client.wait_for_unit("multi-user.target")
         client.succeed(f"""
           ssh -o StrictHostKeyChecking=no {builder.name} \
             'echo hello world on $(hostname)' >&2