Setup ci for gcov test

[nuomi2021]

Maybe we can use https://github.com/marketplace/actions/gcovr-action to generate code coverage reports for PR.

got something like http://coverage.ffmpeg.org/index.vvc_intra.c.5d0b519a39871515a1754ee8847b6d69.html#l678

[frankplow]

I have an old repo where I set up gcov on GitHub Actions, so I can look at porting it maybe. Would you want to test the coverage of the FATE tests or the ffvvc/tests suite? And where would the results be hosted?

[nuomi2021]

The fate test. our target is to make sure fate covers 95% of the code and the most important branch.

And where would the results be hosted?

All action results will be listed here. https://github.com/ffvvc/FFmpeg/actions
You can

1. push the latest code to ffvvc/up
2. cherry-pick 1a3c60a
3. make the change
4. send pr to ffvvc/up
5. we can get the result from https://github.com/ffvvc/FFmpeg/actions

[nuomi2021]

@frankplow , have time for this?
thank you