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

All 263 tests pass, 461 skipped in 14s

1 files ± 0 70 suites ±0 14s ⏱️ - 1h 6m 1s
724 tests ± 0 263 ✅ - 342 461 💤 +351 0 ❌ - 9
864 runs - 1 128 399 ✅ - 1 354 465 💤 +240 0 ❌ - 14

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