Replacing quickpick selection simulation with stubs rather than using…

[gracegoo-stripe]

… sleeps

The sample selection process requires a lot of user inputs, the first 4 of which are quick pick selections. Originally, we kicked off the sample creation then a series of sleeps and a command to select the first item in the quick pick. However, this ends up in race conditions sometimes where we went up getting to the selection command before the quick pick setup is completed or where we end up moving on before the select command is reached. Rather than use timers like this, I updated the test to stub out each call of the quickpick command to return the results we expect. The onCall API of stubs allows us to return different results based on the order it's called.

Example: https://github.com/stripe/vscode-stripe/runs/4360497244?check_suite_focus=true

Getting client  //This is logged from the runtime code when we call the prompt

client quick pick items: html,react  //This is logged from runtime code when we call the showQuickPick API

seleted client undefined // This is logged from the runtime code when the prompt returns

selecting client //This is logged from the simulateSelectAll function.