Use `group_var` argument to `control_lineplot_vars` instead of deprecated `strata` #504

All 614 tests pass, 110 skipped in 1h 6m 12s

1 files ±0 70 suites ±0 1h 6m 12s ⏱️ -3s
724 tests ±0 614 ✅ + 9 110 💤 ±0 0 ❌ - 9
1 992 runs ±0 1 767 ✅ +14 225 💤 ±0 0 ❌ - 14

Results for commit 999b74e. ± Comparison against earlier commit 5400c0d.