Feature Request: Be able to stub out VS code UI in a test (and only a test)

[rchiodo]

I frequently find myself wanting to run a VS code command in a test, but that command may bring up other UI that I need the test to force to a specific value.

It would be super useful if there were some way to stub out parts of VS code ui.

Example:

sinon.stub(window, 'showQuickPick').resolves(myQuickPickItem)

This would likely require changes to VS code to support though.

[connor4312]

Duplicates microsoft/vscode#94746