tests/flat_mutation_reader: add test for partition checksum
tests/flat_mutation_reader: add test for partition checksum

Based on streamed_mutation_test:test_mutation_hash

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