tests/mutation_source_test: generate_mutation_sets() add row that falls into deleted prefix
tests/mutation_source_test: generate_mutation_sets() add row that falls into deleted prefix

This is tailored to the multishard_combining_reader, to make sure it
doesn't loos rows following a range tombstone with a prefix starting
position (whose prefix their keys fall into).

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