tests: add function to provide real network to test case

Very few tests should use it, as it makes the test result somehow
dependent on outside world state. But still, some of them need it.

qubes/tests/__init__.py

@@ -411,6 +411,28 @@ class SystemTestsMixin(object):
             label='black')
         self.app.default_template = template_vm
 
+    def init_networking(self):
+        if not self.app.default_template:
+            self.skipTest('Default template required for testing networking')
+        default_netvm = self.host_app.default_netvm
+        if default_netvm is None:
+            self.skipTest('Default netvm required')
+        if not default_netvm.is_running():
+            self.skipTest('Default netvm required to be running')
+        # Add NetVM stub to qubes-test.xml matching the one on host.
+        # Keeping 'qid' the same is critical because IP addresses are
+        # calculated from it.
+        # Intentionally don't copy template (use default), as it may be based
+        #  on a different one than actually testing.
+        netvm_clone = self.app.add_new_vm(default_netvm.__class__,
+            qid=default_netvm.qid,
+            name=default_netvm.name,
+            uuid=default_netvm.uuid,
+            label=default_netvm.label,
+            provides_network=True
+        )
+        self.app.default_netvm = netvm_clone
+
     def reload_db(self):
         self.app = qubes.Qubes(qubes.tests.XMLPATH)