test/boost/flat_mutation_reader_test: more reversed reader tests
test/boost/flat_mutation_reader_test: more reversed reader tests

Check that the reverse reader emits a stream identical to that emitted
by a reader reading in native order from a table with reversed
clustering order.

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