Change constructions to assertions in structured broadcast tests

[jishnub]

The broadcast operation should already be returning the correct type, and we may type-assert the result instead of constructing an instance from it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.73%. Comparing base (9d139f5) to head (d1f2ef7).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1330   +/-   ##
=======================================
  Coverage   93.73%   93.73%           
=======================================
  Files          34       34           
  Lines       15748    15750    +2     
=======================================
+ Hits        14762    14764    +2     
  Misses        986      986           

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