testing: update BQML training option (#1943)

This updates tests to use `max_iterations` rather than `max_iteration`
which was an alpha option.

Related: b/344469351

Author: shollyman

samples/tests/conftest.py

@@ -162,7 +162,7 @@ def model_id(client: bigquery.Client, dataset_id: str) -> str:
     CREATE MODEL `{}`
     OPTIONS (
         model_type='linear_reg',
-        max_iteration=1,
+        max_iterations=1,
         learn_rate=0.4,
         learn_rate_strategy='constant'
     ) AS (