Improve tests

[Flamefire]

• Add reproducer for fstream reuse on Windows causes a null pointer access violation #150 (reopening an ofstream after close())
• Skip flaky iostream integration test
• Avoid compiling and running test_traits 2 times by merging the boost and std filesystem parts into the same binary
• Enable c++14/17 explicitly on Appveyor cmake-tests to make sure e.g. the traits test are run with those testing the std::filesystem parts
• Refactoring (use nullptr, remove unused private member function)
• Fix return value of filebuf::sync() when fflush fails

Closes #150

[codecov]

Codecov Report

Merging #151 (b0f2767) into develop (1e6badc) will increase coverage by 0.00%.
The diff coverage is 97.33%.

@@           Coverage Diff            @@
##           develop     #151   +/-   ##
========================================
  Coverage    99.33%   99.34%           
========================================
  Files           33       33           
  Lines         3021     3051   +30     
========================================
+ Hits          3001     3031   +30     
  Misses          20       20           

Impacted Files	Coverage Δ
test/test_traits.cpp	100.00% <ø> (ø)
include/boost/nowide/filebuf.hpp	94.16% <90.47%> (+0.02%)	⬆️
include/boost/nowide/args.hpp	95.83% <100.00%> (ø)
include/boost/nowide/stackstring.hpp	100.00% <100.00%> (ø)
include/boost/nowide/utf/convert.hpp	100.00% <100.00%> (ø)
test/test_fstream_special.cpp	100.00% <100.00%> (ø)
test/test_iostream.cpp	100.00% <100.00%> (ø)
test/test_ofstream.cpp	100.00% <100.00%> (ø)
test/test_stackstring.cpp	100.00% <100.00%> (ø)
test/test_stdio.cpp	100.00% <100.00%> (ø)
... and 2 more