QUrl test: ensure that hostnames with bad non-IDN domains are caught too

Leading and double dots are bad, but trailing dots are fine. The ASCII
part of a hostname is supposed to be LDH (letters, digits, hyphen) only,
but we accept '_' (underscore) as an exception too.

Change-Id: I79957ddec4da78a0e2357fe50c8687db03e1c99e
Reviewed-by: David Faure (KDE) <faure@kde.org>