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

tests/gtest/task_project_test.cpp

@@ -312,7 +312,9 @@ TEST_F(TaskProjectLoadTest, LoadingValidProjectSucceeds)
 
 	olive::Project *loaded = task.GetLoadedProject();
 	ASSERT_NE(loaded, nullptr);
-	EXPECT_EQ(loaded->filename(), path);
+	// Project::set_filename() stores native separators on Windows
+	EXPECT_EQ(QDir::fromNativeSeparators(loaded->filename()),
+			  QDir::fromNativeSeparators(path));
 	EXPECT_FALSE(loaded->nodes().isEmpty());
 
 	delete loaded;