Unify JVM and JS tests

[fedefernandez]

Fixes #44

Please @juanpedromoreno could you review? Thanks

[codecov-io]

Codecov Report

Merging #115 into master will decrease coverage by 0.31%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #115      +/-   ##
==========================================
- Coverage   87.79%   87.47%   -0.32%     
==========================================
  Files          36       36              
  Lines         516      519       +3     
  Branches        3        3              
==========================================
+ Hits          453      454       +1     
- Misses         63       65       +2

Impacted Files	Coverage Δ
...b4s/shared/src/main/scala/github4s/Implicits.scala	33.33% <0%> (-8.34%)	⬇️
...4s/shared/src/main/scala/github4s/api/Issues.scala	76.92% <0%> (+6.92%)	⬆️

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[juanpedromoreno]

This is great, oustanding work!