ws: Put cockpit-testing.socket and service in right selinux domain

So that there's less of a difference between what we're running
in our tests and in real life.

Reviewed-by: Patrick Uiterwijk <puitrwijk@redhat.com>
test: Set the selinux file contexts correctly during tests

 * Use correct package names for fixfiles in cockpit-selinux %post
 * Install cockpit-selinux last

Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>
ws: Update selinux policy for a few more cockpit-session things

Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>
ws: Don't access /root/.ssh/known_hosts from cockpit-ws

Selinux audit logs caught this. We really shouldn't be involved
with the /root homedir at all.

Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>

Closed #610