chore: Move polars_session.py to bigframes/testing module (#1760)

Author: TrevorBergeron

tests/unit/polars_session.py renamed to bigframes/testing/polars_session.py

@@ -27,7 +27,7 @@
 # All tests in this file require polars to be installed to pass.
 @pytest.fixture(scope="module")
 def polars_session():
-    from . import polars_session
+    from bigframes.testing import polars_session
 
     return polars_session.TestSession()