Tweak DST tests

[dfarr]

Moves the random failure down to local execution and only throws an exception after we create the durable promise. This prevents the possibility of failure before creating the promise (which may already be completed) which can result in non-progressing executions.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (468eeee) 61.92% compared to head (188910a) 62.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
+ Coverage   61.92%   62.45%   +0.52%     
==========================================
  Files          18       20       +2     
  Lines         851      863      +12     
  Branches      222      225       +3     
==========================================
+ Hits          527      539      +12     
  Misses        296      296              
  Partials       28       28              

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