adding `ard_survival_survfit_diff()` function #3

All 44 tests pass, 20 skipped in 9s

1 files ±0 64 suites +1 9s ⏱️ ±0s
64 tests +1 44 ✅ ±0 20 💤 +1 0 ❌ ±0
181 runs +1 149 ✅ ±0 32 💤 +1 0 ❌ ±0

Results for commit bdbdf2b. ± Comparison against earlier commit 94b77a8.