QObject-test: Reorganize subprocess.

QObject-test: Reorganize subprocess.

On Windows, the signalbug.exe could not be launched since it is
not next to tst_qobject.exe, which is in one of the
'release', 'debug' subfolders.

Introduce a subdirectory structure similar to that of
the QProcess test and use QFINDTESTDATA to locate it.

Change-Id: Ie8f2ede8cb76f22a908cb77517a74076be11fbb7
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>