The test verify_index_stats_output.js uses waitForIndexBuildToStart expecting it to mean that the failpoint previously set has been hit. However, that is not what waitForIndexBuildToStart does: it simply looks for a currentOp reported operation to be an index build.

I believe verify_index_stats_output.js has been passing (until now) by good fortune of a race. It needs to wait for the failpoint to be hit, for which we have JS test utilities, before expecting to find results in $indexStats.