Most tests in the rt-tests suite change their scheduling policy and so need
the privileges to do so.
To give these privileges to non-root users you can add the following line to
/etc/security/limits.conf:

	@users	-	rtprio	20

Note that you probably don't want to do that on a system with untrusted users.
See limits.conf(5) and sched_setscheduler(2) for details.