Test Show instance for Duration

[ceedubs]

No description provided.

[codecov-io]

Codecov Report

Merging #2215 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2215      +/-   ##
==========================================
+ Coverage   94.83%   94.85%   +0.01%     
==========================================
  Files         333      333              
  Lines        5789     5789              
  Branches      211      211              
==========================================
+ Hits         5490     5491       +1     
+ Misses        299      298       -1

Impacted Files	Coverage Δ
core/src/main/scala/cats/instances/duration.scala	100% <0%> (+100%)	⬆️

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