etc/avocado.conf: Use /var/tmp instead of /tmp

To avoid problems with /tmp being mounted on a tmpfs in
certain systems.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

etc/avocado.conf

@@ -3,4 +3,4 @@ base_dir = /usr/share/avocado
 test_dir = /usr/share/avocado/tests
 data_dir = /usr/share/avocado/data
 logs_dir = ~/avocado/job-results
-tmp_dir = /tmp
+tmp_dir = /var/tmp/