RPM of PHPUnit version 6.0 are available in remi repository for Fedora ≥ 22 and for Enterprise Linux (CentOS, RHEL...).
This new major version requires PHP ≥ 7.0 and is not backward compatible with previous version, so the package is designed to be installed beside version 5.
dnf --enablerepo=remi install phpunit6
Installation, Enterprise Linux:
yum --enablerepo=remi install phpunit6 php-phpunit-dbunit3
Notice: this tool is an essential component of PHP QA in Fedora. This version and its dependencies are already submitted to review for Fedora's official repository (review #1420381). I also plan an update in Fedora 25 later.