Indices API: Fix wrong search stats groups

This provides a fix to issue #7644. A new Stats object must be created, and not a reference to the retrieved stats, before we can add stats to it. Otherwise, we would keep on adding to the same object on subsequent calls to IndicesStatsResponse#getPrimaries() or IndicesStatsResponse#getTotal().
first letter of the method name should be lowercase, while at it I'd also rename it to something more specific, since the issue number doesn't say much. `testSearchGroupStatsCounts` for instance? And maybe leave a comment with the link to the github issue for clarity.