Run the vscode test suite on CI

[ekmett]

npm run-script test in the extension folder will download Visual Studio Code and run the ext/test test suite inside it.

Similarly, there are some appveyor hooks that appear to do the same thing in the Microsoft/vscode repo.

It can be hooked up in Setup.hs by uncommenting the npm run-script test option, but a better way to avoid heavy downloads client side for normal users would be to add a flag.