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/dialog_editing_test.cpp

@@ -9,6 +9,7 @@
 #include <QGroupBox>
 #include <QLineEdit>
 #include <QStandardPaths>
+#include <QTemporaryDir>
 #include <QTreeWidget>
 #include <QXmlStreamReader>
 #include <QXmlStreamWriter>
@@ -803,8 +804,13 @@ TEST(DialogProjectProperties, OcioValidationTogglesOnInvalidFilename)
 	auto *ocio_edit = dialog.findChild<QLineEdit *>();
 	ASSERT_NE(ocio_edit, nullptr);
 
-	// A bad config path flags the line edit as invalid (red text)
-	ocio_edit->setText(QStringLiteral("/definitely/not/a/config.ocio"));
+	// A bad config path flags the line edit as invalid (red text). Use a
+	// nonexistent file inside a temp dir: a fixed absolute path is not
+	// guaranteed to stay nonexistent on every platform (e.g. an earlier test
+	// may have created parts of it on a writable drive).
+	QTemporaryDir dir;
+	ASSERT_TRUE(dir.isValid());
+	ocio_edit->setText(dir.filePath(QStringLiteral("nonexistent.ocio")));
 	EXPECT_TRUE(ocio_edit->styleSheet().contains(QStringLiteral("red")));
 
 	// Restoring an empty (default) filename clears the error again

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;