[FT] Improve test_assert_pings_during_br_phys_setup_not_lost* tests

The tests ``test_assert_pings_during_br_phys_setup_not_lost*`` check if
the ping is possible between two ports created on physical networks
(VLAN or flat). Now the method that creates the physical bridges on the
OVS agent ``setup_physical_bridges`` is executed once.

The loop to check the ping between the two ports uses a
``ThreadPoolExecutor``, thus it is possible to execute and check several
pings at the same time.

Closes-Bug: #2107775
Change-Id: I14ec444a4aff3d09b0b8fff8d44f6964e8f7eb5d
(cherry picked from commit 7b2d968)

Author: ralonsoh

neutron/tests/functional/agent/test_l2_ovs_agent.py

@@ -302,14 +302,9 @@ def _test_assert_pings_during_br_phys_setup_not_lost(self, provider_net):
         net_helpers.assert_ping(ns_phys, ip_int)
         net_helpers.assert_ping(self.namespace, ip_phys)
 
-        with net_helpers.async_ping(ns_phys, [ip_int]) as done:
+        with net_helpers.async_ping(ns_phys, [ip_int, ip_phys]) as done:
+            self.agent.setup_physical_bridges(self.agent.bridge_mappings)
             while not done():
-                self.agent.setup_physical_bridges(self.agent.bridge_mappings)
-                time.sleep(0.25)
-
-        with net_helpers.async_ping(self.namespace, [ip_phys]) as done:
-            while not done():
-                self.agent.setup_physical_bridges(self.agent.bridge_mappings)
                 time.sleep(0.25)
 
     def test_noresync_after_port_gone(self):