mutation_test: Specify tmp data dir
mutation_test: Specify tmp data dir

Otherwise we attempt to create sstable files under /.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>
Message-Id: <1464618602-1124-1-git-send-email-a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

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