Open your StandaloneVM testqube console from dom0 terminal:
qvm-console-dispvm testqube
Run this command in testqube console:
sudo apt remove --purge qubes-gui-agent
sudo systemctl set-default graphical.target
Then enable debug mode for this testqube using GUI Settings → Advanced tab → “Run in debug mode” or using dom0 terminal qvm-prefs testqube debug True.
And also change Virtualization mode using GUI Settings → Advanced tab → Virtualization to HVM mode or using dom0 terminal qvm-prefs testqube virt_mode hvm.
But clipboard sharing won’t work since it’s handled by qubes-gui-agent.