RPM of PHPUnit version 6.5 are available in remi repository for Fedora ≥ 24 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