tst_qfiledialog: fixed typo causing semi-random completer() failure

tst_qfiledialog: fixed typo causing semi-random completer() failure

completer_data() was attempting to find the first folder which did
not begin with a '.' character under QDir::root() for usage during the
test.

However, a typo caused it to find the first folder even if it _did_
begin with a '.'; unless the first folder returned by entryList()
was ".", in which case no folder would be found.

Change-Id: Ie95f1add797973d06a8d5f2fa32935366a008de5
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>