Build/Test Tools: Switch to PCOV for the coverage report runner.

PCOV is a dedicated coverage reporting tool that performs significantly faster than Xdebug in coverage mode. This reduces the time that tests with coverage enabled take to run by around 50%.

This also removes the HTML report generation which was producing unusable artifacts in excess of 7 GB in size.

Props johnbillion, desrosj, swissspidy

See #64893


git-svn-id: https://develop.svn.wordpress.org/trunk@62409 602fd350-edb4-49c9-b593-d223f7449a82