Fix QFile autotest not to require elevated privilege to pass

Fix QFile autotest not to require elevated privilege to pass

The open test case tests opening a device path on windows.
As this requires privilege elevation, the test fails when running
nmake check from a console.
As the CI machines appear to run tests with admin privileges, first
check opening the device using the windows API. If that succeeds, then
opening using QFile should succeed. If that fails, the opening using
QFile should fail.
(Since Windows vista, members of the "administrators" group do not run
at elevated privilege all the time. The user needs to opt in via a
UAC prompt or "run as administrator". This is similar to using the
sudo command on unix)

Ran the test as administrator and normally. Now passes in both cases.

Change-Id: Ibd7682eceb61e35d4912fa0392df536f4331f6ed
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>