[UT] Create network to make lazy loading in the models_v2 possible

In the test_port_deletion_prevention_handles_missing_port test which is
part of the classes:
* neutron.tests.unit.extensions.test_l3.L3NatDBSepTestCase
* neutron.tests.unit.extensions.test_extraroute.ExtraRouteDBSepTestCase

it is needed to call pecan application to e.g. create network to make
all models to be ready. Otherwise things like port_forwarding attribute
in the Port class, which is loaded in lazy mode isn't available and test
was failing when was run in the isolated environment.
It wasn't failing in the gate as there were other tests run by the same
worker before this one and then it was all initialized properly.

Closes-Bug: #2028285
Change-Id: Ie2382540d7c0a8813f093ddf51d82fe530026d71
(cherry picked from commit 14b2f4f)

Author: slawqo

neutron/tests/unit/extensions/test_l3.py

@@ -4473,9 +4473,15 @@ class L3NatDBSepTestCase(L3BaseForSepTests, L3NatTestCaseBase,
 
     def test_port_deletion_prevention_handles_missing_port(self):
         pl = directory.get_plugin(plugin_constants.L3)
-        self.assertIsNone(
-            pl.prevent_l3_port_deletion(context.get_admin_context(), 'fakeid')
-        )
+        # NOTE(slaweq): it's needed to make at least one API call to the
+        # application to initialize all models which are using lazy loading of
+        # some attributes,
+        # check https://bugs.launchpad.net/neutron/+bug/2028285 for details
+        with self.network():
+            self.assertIsNone(
+                pl.prevent_l3_port_deletion(context.get_admin_context(),
+                                            'fakeid')
+            )
 
 
 class L3TestExtensionManagerWithDNS(L3TestExtensionManager):