tests: add clues to find missing private/qtobject_p.h [skip ci]
Finding all the dependencies missing from my Fedora system to run the whole
test suite was relatively quick - except for this one.
#if QT_VERSION > 0x050000
// include some random private headers
+// As you're not supposed to use it, your system may miss
+// qobject_p.h. To locate it try one of these commands:
+// - dnf provides */private/qobject_p.h
+// - apt-file search qobject_p.h
#include <private/qobject_p.h>
#endif
