fix: clear pytest recwarn in MutationsBatcher tests

Author: mutianf

packages/google-cloud-bigtable/tests/unit/data/_async/test_mutations_batcher.py

@@ -1058,6 +1058,7 @@ async def test__on_exit(self, recwarn):
                 assert len(w) == 1
                 assert "unflushed mutations" in str(w[0].message).lower()
                 assert str(num_left) in str(w[0].message)
+            recwarn.clear()
             # calling while closed is noop
             instance._closed.set()
             instance._on_exit()

packages/google-cloud-bigtable/tests/unit/data/_sync_autogen/test_mutations_batcher.py

@@ -923,6 +923,7 @@ def test__on_exit(self, recwarn):
                 assert len(w) == 1
                 assert "unflushed mutations" in str(w[0].message).lower()
                 assert str(num_left) in str(w[0].message)
+            recwarn.clear()
             instance._closed.set()
             instance._on_exit()
             assert len(recwarn) == 0