travis: check the --empty-ns option

Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Looks-good-to: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

scripts/travis/travis-tests

@@ -88,6 +88,8 @@ ip net add test
 ./test/zdtm.py run -t zdtm/static/env00 --rpc		# Basic
 ./test/zdtm.py run -t zdtm/static/ptrace_sig -f h --rpc # Error handling (crfail test)
 
+./test/zdtm.py run --empty-ns -T zdtm/static/socket-tcp*-local --iter 2
+
 pip install flake8
 make lint