add std::filesystem to tests

[ObiWahn]

filesystem

• [x] linux - gcc
• [x] linux - clang
• [x] windows
• [ ] mac

problems

Maybe MacOS 10.15 is required.

• https://stackoverflow.com/questions/54786973/what-library-to-link-to-for-stdfilesystem-with-xcode/54834871#54834871

[codecov[bot]]

Codecov Report

Merging #26 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #26   +/-   ##
=======================================
  Coverage   98.91%   98.91%           
=======================================
  Files          23       23           
  Lines         736      736           
=======================================
  Hits          728      728           
  Misses          8        8

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