Add a test for QFilePrivate::fileName offset on Linux

This extends the test suite introduced in 497f0af1f7 for
a known-to-be-good case.

Change-Id: Ib574ecfdb8e9d91985d6df8c092896581d1d06ff
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>