update `strata` var to `group_var` #3110

All 263 tests pass, 438 skipped in 16s

1 files ± 0 69 suites - 1 16s ⏱️ - 1h 8m 20s
701 tests - 23 263 ✅ - 320 438 💤 +328 0 ❌ - 31
841 runs - 1 153 399 ✅ - 1 325 442 💤 +217 0 ❌ - 45

Results for commit f8a7457. ± Comparison against earlier commit ae7e067.