test: ignore bigframes/testing folder from testing (#1767)

This is to disable failures like the following:

_____________ ERROR collecting bigframes/testing/polars_session.py _____________
bigframes/testing/polars_session.py:19: in <module>
    import polars
E   ModuleNotFoundError: No module named 'polars'

Author: shobsi

noxfile.py

@@ -429,6 +429,8 @@ def doctest(session: nox.sessions.Session):
             "third_party/bigframes_vendored/ibis",
             "--ignore",
             "bigframes/core/compile/polars",
+            "--ignore",
+            "bigframes/testing",
         ),
         test_folder="bigframes",
         check_cov=True,