tls: make test-server runtime dir in /dev/shm
tls: make test-server runtime dir in /dev/shm

Our unit tests container, when run on GitHub actions runners, gets a
overlayfs for /tmp, which doesn't support `O_TMPFILE`.  Since we depend
on `O_TMPFILE`, we need to make sure this is on a real tmpfs (as it will
always be at runtime).

/dev/shm is a bit of a weird place for it, but it will do the job quite
nicely.

martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit