tests/flat_mutation_reader: add test for multi range reader
tests/flat_mutation_reader: add test for multi range reader

Based on mutation_reader.cc:test_multi_range_reader.

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