sstable_datafile_tests: Replace '---' with auto
sstable_datafile_tests: Replace '---' with auto

Fixes compilation issues on some g++.
Message-Id: <ae1b1ce16bc41ae66227efa368a5db71e4c3d12e@scylladb.com>

duarten/scylla,scylladb/scylla,kjniemi/scylla,scylladb/scylla,scylladb/scylla,kjniemi/scylla,avikivity/scylla,raphaelsc/scylla,kjniemi/scylla,duarten/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,raphaelsc/scylla,raphaelsc/scylla,avikivity/scylla