tests/flat_mutation_reader_assertion: add produces_partition()
tests/flat_mutation_reader_assertion: add produces_partition()

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