plan_cache_index_create.js should wait for index build start, rather than just createIndexes command start

The plan_cache_index_create.js test checks currentOp to confirm that an index build is running. Once confirmed it assumes that the createIndexes command has cleared the plan cache. This is an incorrect assumption to make as the createIndexes command will be visible in currentOp on command execution, which is prior to the MultiIndexBlock::init() call which triggers a clearQueryCache() call further down the call stack.

Instead, we should confirm that the hangAfterStartingIndexBuild has been reached. Note that this change may not work for the calls to !indexBuildIsRunning() in this test, which can continue to use currentOp.