test: explicitly use US location for session in tests (#650)

This avoids some warnings we see and ignore in our tests.

It might also address some flakiness in
`tests/system/small/ml/test_llm.py::test_create_text_generator_model`
and
`tests/system/small/ml/test_llm.py::test_create_text_generator_32k_model`,
but the root cause of that flakiness is still TBD.

Author: tswast

tests/system/conftest.py

@@ -128,7 +128,10 @@ def resourcemanager_client(
 
 @pytest.fixture(scope="session")
 def session() -> bigframes.Session:
-    return bigframes.Session()
+    context = bigframes.BigQueryOptions(
+        location="US",
+    )
+    return bigframes.Session(context=context)
 
 
 @pytest.fixture(scope="session")