run test on GitHub Actions for three platforms

[koron]

This PR adds GitHub Actions workflow to run test with three OS: Linux (ubuntu), macOS, and Windows.

go-swagger/go-swagger#2492 (comment) might be prevented if there are tests on multiple platform.

So I created it using GitHub Actions.

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This PR is branched from v0.20.0 for GHA failes on Windows only.
You can see example run here

[codecov]

Codecov Report

Merging #35 (f35a10c) into master (35cd441) will decrease coverage by 3.46%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
- Coverage   82.53%   79.06%   -3.47%     
==========================================
  Files           3        3              
  Lines         166      172       +6     
==========================================
- Hits          137      136       -1     
- Misses         14       21       +7     
  Partials       15       15              

Impacted Files	Coverage Δ
loaders.go	83.72% <0.00%> (-16.28%)	⬇️
options.go	100.00% <0.00%> (ø)
spec.go	72.89% <0.00%> (+0.78%)	⬆️

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[fredbi]

@koron thanks. You should know that at the moment, the full test suite is broken on windows (even though ordinary cases work).
This is due to github.com/go-openapi/spec#97 which I am trying to deal with at the moment (issue with remote circular $ref forming an invalid URI).
Getting to know github actions could be nice, however. I've tried to set up appveyor for macOS but the setting was a mess.