Fix QComboBox icon testing

Fix QComboBox icon testing

The icons used were always null -- the files used to create them were
not present. So

1) add back the files used for the test (qtlogo.png,
copied from src/widgets/dialogs/images/qtlogo-64.png, and qtlogoinverted.png,
its copy rotated by 180°);

2) use QFINDTESTDATA to get the paths to the files.

Change-Id: I26f094079e037116e49e2a95345c2dea7496eecb
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>