Add code coverage tracking

[pento]

To help track what we're testing, code coverage is useful.

Let's investigate the services that can provide this.

[pento]

46373.diff​ gets codecov (kind of) working.

There are some unit test failures when it runs, though: ​https://travis-ci.org/WordPress/wordpress-develop/jobs/499749838

Results: ​https://codecov.io/gh/WordPress/wordpress-develop/branch/try%2Fcodecov

@whyisjake, @joyously: y'all were going to look at this, here's a starting point. You should be able to connect services to your personal GitHub accounts for testing, but let me know if you need something connected to the WordPress organisation.

[whyisjake]

Accidentally started on this yesterday with a PR to core...

​https://github.com/WordPress/wordpress-develop/pull/42

Thanks for kicking this off @pento.

[johnbillion]

Coverage reports in PHPUnit require Xdebug, which we disable on Travis for performance reasons. Do these coverage reports actually work?

[pento]

46373.diff​ uses phpdbg instead of xdebug, because it's a whole lot faster.

[jorbin]

I’m removing from 5.2, but if there is traction this could come back in as a tool chain request like this can happen at just about any time.

[desrosj]

Somewhat related: #39265, #39237.

The patch needs a refresh to account for the tests now being executed within the local Docker environment.

[netweb]

Replying to jorbin:

I’m removing from 5.2, but if there is traction this could come back in as a tool chain request like this can happen at just about any time.

Same, I’m removing from 5.3, but if there is traction this could come back in as a tool chain request like this can happen at just about any time.