Fixes a currently failing code coverage job because the PHPUnit config is outdated.

See #6035

The PHP 5.6 failures are known, see #6018

@wp-cli/committers friendly ping

Is this related warning intentional?

 Warning - The configuration file did not pass validation!
  The following problems have been detected:

  Line 21:
  - Element 'coverage': This element is not expected.

  Test results may not be as expected.

I see we merged similar PRs already, just curious

Yes, that's expected because those tests use an older PHPUnit version due to the older PHP version.