Minor Description
Currently IndexPlanner returns a plan only if if any of the config is applicable for the query being eecuted. While running Lucene testcases current approach was to always return a plan (in LowCostLuceneIndexProvider with a very small cost.
To support similar thing with IndexPlanner we need to add a testMode flag. If that flag is set the planner would ensure that for given test run it should always return a plan. If no plan is possible then it would fail with exception