test: Use RPM name cockpit-selinux-policy to be clearer

Since we'll start distributing the selinux policy, we want to
give it a good name. cockpit-selinux might be used for selinux
functionality, so use cockpit-selinux-policy.

Also mark our policy RPM as noarch

Closes #963
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>