pkg/sensors/tracing: fix enforcer security tests

Because of root-images update in previous commit, the enforcer security
tests started failing, after investigation we realized that the
difference was that the old image mounted /tmp on the disk fs and the
new one with tmpfs. Then the direct-write-tester.c program was failing
using O_DIRECT (because apparently it fails on tmpfs). It failed
silently on recent linux versions however because O_DIRECT on tmpfs
might just be a noop.

/var/tmp should be backed by disk so it should be a fix for our issue.

Signed-off-by: Mahe Tardy <mahe.tardy@gmail.com>