Fix wxRmdir() with non-ASCII paths

Don't apply at best unnecessary, and actually harmful, as it uses a wrong
conversion, fn_str() when calling wxRmDir() which takes wxString.

Update unit tests to check that wxRmdir() now works with non-ASCII filenames
too.

Closes #17644.

src/common/filefn.cpp

@@ -1192,7 +1192,7 @@ bool wxRmdir(const wxString& dir, int WXUNUSED(flags))
 #if defined(__VMS__)
     return false; //to be changed since rmdir exists in VMS7.x
 #else
-    if ( wxRmDir(dir.fn_str()) != 0 )
+    if ( wxRmDir(dir) != 0 )
     {
         wxLogSysError(_("Directory '%s' couldn't be deleted"), dir);
         return false;