Remove use of deprecated 'channel' argument. (#6271)

Instead, just use mocks and a custom iterator to bypass the underlying
GAPIC client altogether.

firestore/tests/unit/test_document.py

@@ -519,19 +519,28 @@ def test_get_not_found(self):
             [document], field_paths=field_paths, transaction=None)
 
     def _collections_helper(self, page_size=None):
-        from google.api_core import grpc_helpers
+        from google.api_core.page_iterator import Iterator
+        from google.api_core.page_iterator import Page
         from google.cloud.firestore_v1beta1.collection import (
             CollectionReference)
         from google.cloud.firestore_v1beta1.gapic.firestore_client import (
             FirestoreClient)
-        from google.cloud.firestore_v1beta1.proto import firestore_pb2
+
+        class _Iterator(Iterator):
+
+            def __init__(self, pages):
+                super(_Iterator, self).__init__(client=None)
+                self._pages = pages
+
+            def _next_page(self):
+                if self._pages:
+                    page, self._pages = self._pages[0], self._pages[1:]
+                    return Page(self, page, self.item_to_value)
 
         collection_ids = ['coll-1', 'coll-2']
-        list_coll_response = firestore_pb2.ListCollectionIdsResponse(
-            collection_ids=collection_ids)
-        channel = grpc_helpers.ChannelStub()
-        api_client = FirestoreClient(channel=channel)
-        channel.ListCollectionIds.response = list_coll_response
+        iterator = _Iterator(pages=[collection_ids])
+        api_client = mock.create_autospec(FirestoreClient)
+        api_client.list_collection_ids.return_value = iterator
 
         client = _make_client()
         client._firestore_api_internal = api_client
@@ -550,12 +559,11 @@ def _collections_helper(self, page_size=None):
             self.assertEqual(collection.parent, document)
             self.assertEqual(collection.id, collection_id)
 
-        request, = channel.ListCollectionIds.requests
-        self.assertEqual(request.parent, document._document_path)
-        if page_size is None:
-            self.assertEqual(request.page_size, 0)
-        else:
-            self.assertEqual(request.page_size, page_size)
+        api_client.list_collection_ids.assert_called_once_with(
+            document._document_path,
+            page_size=page_size,
+            metadata=client._rpc_metadata,
+        )
 
     def test_collections_wo_page_size(self):
         self._collections_helper()