Add ref_group_last for custom split funs #2666

All 767 tests pass, 20 skipped in 1m 26s

      1 files     81 suites 1m 26s ⏱️
  787 tests   767 ✔️   20 💤 0 ❌
1 676 runs 1 042 ✔️ 634 💤 0 ❌

Results for commit 80a81e6.