test: ignore bigframes/testing folder from testing

[shobsi]

This is to disable failures like the following:

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

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Fixes #<issue_number_goes_here> 🦕

[tswast]

I mailed #1775 as an alternative. This makes sense as a quick fix, though.