[DOC] Add VS code-server checks to testing checklist

[kcpevey]

We've had 2 issues recently where vs code updates caused regressions. vscode is a major part of how many devs work on qhub. For me personally, I can't/won't use qhub as a development platform without it. Since its so important, I think we should add some release checks.

Perhaps:

• VS Code launches correctly
• Python extension installs
• Basic python file can be run through the vs code ui
• Basic python file can be run through the terminal.

[trallard]

@iameskild since this is pertaining to the testing & release checklist will assign to you