Add tests for flat_mutation_reader utils
Add tests for flat_mutation_reader utils

Test flat_mutation_reader_from_mutations and
read_mutation_from_flat_mutation_reader.

Signed-off-by: Piotr Jastrzebski <2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>

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