[T2 aiotest] network (#11729)

* add aio test

* enable network watcher test

sdk/network/azure-mgmt-network/tests/_aio_testcase.py

@@ -0,0 +1,33 @@
+import asyncio
+from unittest.mock import Mock
+
+from azure.core.credentials import AccessToken
+from azure.identity.aio import DefaultAzureCredential
+from devtools_testutils import AzureMgmtTestCase
+
+class AzureMgmtAsyncTestCase(AzureMgmtTestCase):
+
+    def setUp(self):
+        super(AzureMgmtAsyncTestCase, self).setUp()
+
+    @property
+    def event_loop(self):
+        return asyncio.get_event_loop()
+
+    def create_mgmt_aio_client(self, client, **kwargs):
+        if self.is_live:
+            credential = DefaultAzureCredential()
+        else:
+            credential = Mock(get_token=asyncio.coroutine(lambda _: AccessToken("fake-token", 0)))
+        return client(
+            credential=credential,
+            subscription_id=self.settings.SUBSCRIPTION_ID
+        )
+
+    def to_list(self, ait):
+        async def lst():
+            result = []
+            async for item in ait:
+                result.append(item)
+            return result
+        return self.event_loop.run_until_complete(lst())