[windows] copy dlls in tests build folder

[StRigaud]

... so that tests can be run without copying manually the dlls or adding them to the path.

This only concern windows. And it was not yet tests.

closes #250

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.34%. Comparing base (437bcd3) to head (69c2833).
Report is 207 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #314      +/-   ##
==========================================
- Coverage   50.09%   47.34%   -2.75%     
==========================================
  Files          15      235     +220     
  Lines        3222     3128      -94     
  Branches     1364     1550     +186     
==========================================
- Hits         1614     1481     -133     
+ Misses        532      419     -113     
- Partials     1076     1228     +152     

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[haesleinhuepf]

I tested this on my Windows machine and it works!!

Big thanks @StRigaud