tests: fix partitioner_test build on gcc 5
tests: fix partitioner_test build on gcc 5

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