tests: sstables: Use read_row() for single-key reads
tests: sstables: Use read_row() for single-key reads

So that as_mutation_reader() will create the same kind of reader which
database::make_sstable_reader() does.

Before this change, all readers were range readers.

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