Add tst_dropArea

Removed the requirement to have a parent, it's not really needed,
conceptually.

tests/tst_droparea.cpp

@@ -0,0 +1,20 @@
+/*
+  This file is part of KDDockWidgets.
+
+  SPDX-FileCopyrightText: 2019-2022 Klarälvdalens Datakonsult AB, a KDAB Group company <info@kdab.com>
+  Author: Sérgio Martins <sergio.martins@kdab.com>
+
+  SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only
+
+  Contact KDAB at <info@kdab.com> for commercial licensing options.
+*/
+
+#include "main.h"
+#include "controllers/DropArea.h"
+#include "Platform.h"
+
+TEST_CASE("DropArea CTOR")
+{
+    // Tests that ctor runs and doesn't leak
+    Controllers::DropArea da(nullptr, {});
+}