[🚀 Request] Allow to generate extension test coverage using c8 plugin

[djelinek]

Describe the request

generate test coverage reports for extension tested by ExTester

https://www.npmjs.com/package/c8

Usage

e38708b

[djelinek]

Hi @TamiTakamiya We can continue on this issue with your work on test coverage.

FYI I was looking for reason why the --extensionDevelopmentPath flag was removed from ExTester API. The reason is simple, in past we have decided to chose path where we are executing tests against packed VSIX extension and not the run from source in runtime.

I would like to do not mix these to approaches by adding mentioned flag back at the moment.

But it should be possible to provide new ExTester flag - eg --coverage which will do whole magic in ExTester backend without changes WDYT?

[TamiTakamiya]

@djelinek Thanks for the updates. I started looking at this again with the latest code base.

[TamiTakamiya]

I was thinking of using nyc instead of c8 simply because we have been using nyc in our project. However, I got to know that Microsoft's vscode-test-cli now supports code coverage through c8 and it is supporting the --coverage flag that @djelinek suggested above. I will take a look at their code and consider how ExTester code can be modified to support the flag.

[TamiTakamiya]

Draft PR #1227 was created. I will add some more refinements and send it for review. Thanks.