travis: allow to run up to two tests in parallel

It works faster and checks that two criu processes can work concurrently.
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

scripts/travis/travis-tests

@@ -18,7 +18,7 @@ make -C test/zdtm
 umask 0000
 export SKIP_PREP=1
 
-./test/zdtm.py run -a -x 'cgroup*'
+./test/zdtm.py run -a -x 'cgroup*' -p 2
 
 bash ./test/jenkins/criu-fault.sh
 bash ./test/jenkins/criu-fcg.sh