test: Check negotiate authentication
selinux: cockpit-session uses GSSAPI which wants to write to tmpdir

GSSAPI writes to a replay cache. cockpit-session now uses GSSAPI
and needs to be able to write, read, delete from tmpdir

Symptom:

cockpit-session: gssapi auth failed: Unspecified GSS failure.
	Minor code may provide more information (Cannot create replay
	cache file /var/tmp/HTTP_0: File exists)
HACKING: Add instructions for how to configure SSO

This is a bit convoluted currently, but it's important to make this
reproducable and for people to be able to develop against it ... or
it will never ever get better.

Closes #1124