Add ref_group_last for custom split funs #1657

All 769 tests pass, 20 skipped in 1m 53s

      1 files ±0     81 suites ±0 1m 53s ⏱️ +4s
  789 tests ±0   769 ✔️ ±0   20 💤 ±0 0 ❌ ±0
1 677 runs ±0 1 043 ✔️ ±0 634 💤 ±0 0 ❌ ±0

Results for commit c24b984. ± Comparison against earlier commit 6ad964a.