fix: honor DirectoryIsValid create flag; unbreak Windows CI tests

DirectoryIsValid() ignored try_to_create_if_not_exists and always
called mkpath(). On the Windows CI runner (which may create dirs at
the drive root) PathWidget validation both created bogus directories
and never flagged invalid paths; on Linux the mkpath just failed.

- filefunctions: only mkpath when try_to_create_if_not_exists is set
- TaskProjectLoadTest: Project::set_filename() stores native
  separators on Windows, normalize before comparing paths
- DialogProjectProperties: use a nonexistent file inside a temp dir
  instead of a fixed /definitely/... path that prior tests may have
  partially created on a writable drive

app/common/filefunctions.cpp

@@ -168,7 +168,8 @@ bool FileFunctions::DirectoryIsValid(const QDir &d,
 									 bool try_to_create_if_not_exists)
 {
 	// Return whether the directory exists, or whether it could be created if it doesn't
-	return d.exists() || d.mkpath(QStringLiteral("."));
+	return d.exists() ||
+		   (try_to_create_if_not_exists && d.mkpath(QStringLiteral(".")));
 }
 
 QString FileFunctions::EnsureFilenameExtension(QString fn,