tests/mutation_reader: test fast forwarding combined reader
tests/mutation_reader: test fast forwarding combined reader

Signed-off-by: Paweł Dziepak <e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>

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