tst_QUrl: check that prohibited characters in hostnames are not valid

qt_nameprep is tested by tst_qurlinternal. We just need to be sure that
QUrl handles them correctly.

Change-Id: Ic563004870d2cf2fa7a31ce49fff7280d5ffb5f3
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>