tests/sstable_datafile_test: Fix regression
tests/sstable_datafile_test: Fix regression

This patch fixes a regression introduced in 9e88b60, where the wrong
clustering key was being specified.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>
Message-Id: <20170509091621.2682-1-a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

duarten/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla