software/erp5/test: fix ResourceWarning on python3

 - use the requests.Session in a context manager so that resources are
properly closed.
 - close the stdout of caucase process we want to keep silent

See merge request nexedi/slapos!1075

software/erp5/test/test/test_balancer.py

@@ -110,6 +110,7 @@ class CaucaseService(ManagedResource):
             '--netloc', backend_caucased_netloc,
             '--service-auto-approve-count', '1',
         ],
+        # capture subprocess output not to pollute test's own stdout
         stdout=subprocess.PIPE,
         stderr=subprocess.STDOUT,
     )
@@ -127,6 +128,7 @@ class CaucaseService(ManagedResource):
     # type: () -> None
     self._caucased_process.terminate()
     self._caucased_process.wait()
+    self._caucased_process.stdout.close()
     shutil.rmtree(self.directory)
 
 
@@ -525,7 +527,7 @@ class TestHTTP(BalancerTestCase):
   def test_keep_alive(self):
     # type: () -> None
     # when doing two requests, connection is established only once
-    session = requests.Session()
+    with requests.Session() as session:
       session.verify = False
 
       # do a first request, which establish a first connection
@@ -539,6 +541,7 @@ class TestHTTP(BalancerTestCase):
       new_conn.assert_not_called()
 
       parsed_url = six.moves.urllib.parse.urlparse(self.default_balancer_url)
+
       # check that we have an open file for the ip connection
       self.assertTrue([
           c for c in psutil.Process(os.getpid()).connections()

software/erp5/test/test/test_erp5.py

@@ -64,7 +64,7 @@ class TestPublishedURLIsReachableMixin(object):
     # erp5 site is not created, with 500 when mysql is not yet reachable, so we
     # configure this requests session to retry.
     # XXX we should probably add a promise instead
-    session = requests.Session()
+    with requests.Session() as session:
       session.mount(
           base_url,
           requests.adapters.HTTPAdapter(