Test kf/FilenameUtils.h

[belyshevdenis]

• Write tests for all functions of include/kf/FilenameUtils.h using kmtest, see test/MapTest and test/Vector.cpp for example
• keep the same coding style
• Tests can't be run, so just check that they are compiled successfully
• The code can't use c++ exceptions

[SergiusTheBest]

@belyshevdenis I think it's better to add documentation first, so copilot won't be so confused.