Source: Enhance check for C++17 filesystem support

* Add `lexically_normal` test for all platforms.

* On Windows, MinGW does not currently handle `lexically_normal()`
  correctly on UNC path, but MSVC and IntelLLVM do--add a comment
  on this to avoid future confusion.

* Add test with `\\?\` notation and `weakly_canonical` that also triggers
  the MinGW bug, but is fine with MSVC and oneAPI, for a more robust and
  comprehensive test.