tests: upgrade the "buffer_overflow" test to flat_mutation_reader_v2
tests: upgrade the "buffer_overflow" test to flat_mutation_reader_v2

Signed-off-by: Michael Livshin <ae60993c65de9cdf2ca07b7426c976808535d555@scylladb.com>

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