QFileSystemModel: report special files which are not symlinks

Since special files have file size == -1, they were always filtered
out by QFileSystemModel, even when passing QDir::System as filtering
option. Keep them instead.

The testcase is more convoluted than it should be because QFSM
is so broken that it returns valid indexes for invisible elements
in the model (such as filtered out elements).

Change-Id: I023a9813dbfeed7be99dded42c66b1191afdc17e
Task-number: QTBUG-20968
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>