ser: cleanup tests

[davidhewitt]

Removes the macro to follow a similar strategy to de.rs.

[codecov-io]

Codecov Report

Merging #6 (2954757) into master (566c870) will increase coverage by 1.37%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
+ Coverage   72.69%   74.06%   +1.37%     
==========================================
  Files           4        4              
  Lines         509      536      +27     
==========================================
+ Hits          370      397      +27     
  Misses        139      139              

Impacted Files	Coverage Δ
src/ser.rs	81.15% <100.00%> (+2.82%)	⬆️

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