Make timing_stress_test options for page split race conditions work.

The config name for engaging the timing_stress for page split are "split_race_1, split_race_2, split_race_3 etc., as per dist/api_data.py and the same is being used by test/format as well.

But the __wt_timing_stress_config() is checking for the configuration parameters with name "split_1, split_2, split_3" Hence our timing stress is not execising the race conditions check for page split.