Windows: Do not print test output to both stdout and system debug log

Avoid piping the test output to both the Windows system debugger, and
stdout. This fixes duplicate output in Qt Creator, which displays messages
from both sources.

[ChangeLog][QtTestLib] The (default) plain text logger on Windows now
logs to either the system debug log (in case no console is open), or
stdout, not both.

Task-number: QTBUG-34630
Change-Id: I35fe9f4a50cc660d79fad7dffa6d19659b2102ae
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>