Fix LogRankTest variance computation

[lrnv]

Correct the variance computation in the LogRankTest to prevent inflated test statistics when groups differ, resolving issue #85.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.15%. Comparing base (933e88f) to head (ea354e1).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #86      +/-   ##
==========================================
+ Coverage   92.12%   92.15%   +0.03%     
==========================================
  Files          12       12              
  Lines         978      982       +4     
==========================================
+ Hits          901      905       +4     
  Misses         77       77              

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