PHPUnit: add clover code coverage config to config file

[jrfnl]

This moves the clover code coverage config from the composer script to the phpunit[10].xml.dist config files.

It also moves the default location for the file from the project root to the tests/coverage/ directory so all coverage files are in the same place (and automatically git ignored etc).

Includes updating the "upload to CodeCov" step to find the file in the new location.