add more tests for kaplan-meier widget and fix median survival time

[JakaKokosar]

Issue

Description of changes

Includes

• [ ] Code changes
• [X] Tests
• [ ] Documentation

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@fb41273). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master       #5   +/-   ##
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  Coverage          ?   94.10%           
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  Files             ?        2           
  Lines             ?      339           
  Branches          ?        0           
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  Hits              ?      319           
  Misses            ?       20           
  Partials          ?        0           

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