onDidOpenNotebookDocument - emit event only once when opened in two editors

1) Notebook Document
       onDidOpenNotebookDocument - emit event only once when opened in two editors:
     Error: Timeout of 60000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves. (D:\a\_work\1\s\extensions\vscode-api-tests\out\singlefolder-tests\notebook.document.test.js)
      at listOnTimeout (node:internal/timers:559:17)
      at processTimers (node:internal/timers:502:7)

https://dev.azure.com/vscode/VSCode/_build/results?buildId=70483&view=logs&j=a8a6689c-5a84-59c0-482a-7dd9f21eb2d8&t=1fa6fb8d-7bb3-5acd-5778-bf99aac7f637&l=391

https://dev.azure.com/monacotools/Monaco/_build/results?buildId=180738&view=logs&j=0304f796-6596-5b42-8573-3d5da85bc4a5&t=4c50b82f-3f48-5844-7225-93b0027cedfd&l=390

[bpasero]

For new tests or to see if flakiness is fixed, I was doing this for a few cases:

• setup a branch where potentially flaky tests are enabled again
• duplicate the run test task in the yml files 20 times to have many test runs at once
• trigger a few builds over the day and see if any flake is there

If like a 100 runs are without flake, then I merge to main

[mjbvz]

@rebornix I’m not sure. I’ve bumped the content that the webview tries to load with #157959 but am not sure that will help with these issues