Fix MSVC compiler warning in code snippets

Fixes
 warning C5046: 'testObject::MyTestObject::toString': Symbol involving type with internal linkage not defined

Change-Id: I9925eb15e262f29e636c019c87311a6ea2c47505
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

src/testlib/doc/snippets/code/src_qtestlib_qtestcase.cpp

@@ -158,11 +158,9 @@ QTEST_MAIN(TestQString)
 
 void testObject()
 {
-class MyTestObject: public QObject
-{
-    public:
-        void toString();
-};
+    class MyTestObject : public QObject
+    {
+    };
 //! [18]
 MyTestObject test1;
 QTest::qExec(&test1);