Fix qlogging test for release configuration

Fix qlogging test for release configuration

The helper process 'app' wasn't built in release configuration.

Also improved finding the helper executable to utilize
QFINDTESTDATA and print out a proper errors if it could not be
found or started.

Note that adding ".exe" to process name in Windows is unnecessary
as QProcess already does that for you, so removed the ifdeffing.

Task-number: QTBUG-24330
Change-Id: Ibe75e0ecd24181ab623d0a60f17ecaf92052b0dd
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>