tests: Decouple tst_multisplitter from QWidget more

It now asks the platform to create a test view instead of
using one that inherits from QWidget.

src/qtwidgets/TestHelpers_qtwidgets.cpp

@@ -41,7 +41,7 @@ public:
 
     QSize maxSizeHint() const override
     {
-        return m_opts.maxSize;
+        return m_opts.maxSize.boundedTo(Views::View_qtwidgets<QWidget>::maximumSize());
     }
 
 private: