tests: avoid 'Cannot allocate memory' error in testinfra

tests: avoid 'Cannot allocate memory' error in testinfra

```
------------------------------ Captured log setup ------------------------------
display.py                 174 INFO     Wednesday 13 February 2019  15:54:15 +0000 (0:00:07.787)       0:02:11.607 ****
ansible.py                  61 INFO     RUN Ansible('setup', None, {'check': True, 'become': False}): {'_ansible_no_log': False,
 '_ansible_parsed': False,
 '_ansible_verbose_override': True,
 'changed': False,
 'module_stderr': u'Connection to 192.168.121.87 closed.\r\n',
 'module_stdout': u'bash: /bin/sh: Cannot allocate memory\r\n',
 'msg': u'MODULE FAILURE\nSee stdout/stderr for the exact error',
 'rc': 126}
```

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>