tests/slapproxy: discard child slapos proxy output

to keep tests silent.

On python2, this uses subprocess32, which backports DEVNULL (and
default setting of closing file descriptors).

slapos/tests/slapproxy/__init__.py

@@ -34,7 +34,10 @@ import os
 import logging
 import shutil
 import socket
-import subprocess
+try:
+  import subprocess32 as subprocess
+except ImportError:
+  import subprocess
 import sys
 import tempfile
 import time
@@ -1029,6 +1032,8 @@ database_uri = %(tempdir)s/lib/external_proxy.db
             sys.executable, '%s/../cli/entry.py' % os.path.dirname(slapos.tests.__file__),
             'proxy', 'start', '--cfg', self.external_slapproxy_configuration_file_location
         ],
+        stdout=subprocess.DEVNULL,
+        stderr=subprocess.DEVNULL,
         env={"PYTHONPATH": ':'.join(sys.path)}
     )
     # Wait a bit for proxy to be started