If you want code coverage in your Tests, you can use one of pcov(recommended) or xdebug
To enable pcov put php -dpcov.enabled=1 before your script:
e.g: php -dpcov.enabled=1 vendor/bin/phpunit
!!! to use pcov with phpunit <=7.0 you need pcov/clobber
To enable xdebug got to the Documentation about xdebug.md.