Add test for red stop button on Shiny app

[juliasilge]

Related to:

• Add a button to stop a terminal process from the Viewer pane #10625
• Update Shiny and Quarto extensions for new stop button affordance #10753
• Add a big red stop button for Positron's Viewer pane shiny-vscode#103

The Shiny VS Code extension was just updated with the new updated Positron-related, so we can now update the bootstrapped version and add a test that the button works on a Shiny app. I do think it's worth writing a test here, because they can't test Positron-related functionality in the extension.