Windows: Do not pipe output to both stdout and system debugger buffer

When a test is launched on Windows (plain text logger), test output is both piped into the system debug buffer and to stdout. This leads to Qt Creator printing the same messages twice ...

Ideally TestLib would write to the system debug buffer only if stdout is not available (GUI application). If that's not possible, a workaround might be an option / environment variable so that Qt Creator can explicitly limit the output to one destination.