Update test case to use valid custom WiredTiger configuration values

The noPassthrough/wt_index_option_defaults.js test sets the WiredTiger table configuration split_pct to values less than 50. A recent change in WT-2439 put a lower bound of 50 on that configuration option. The goal of the test case is to ensure custom configuration values appear where expected, the actual values aren't material. It should be updated to use a different set of values.