test: flat_mutation_reader_test: test_reverse_reader_is_mutation_source: prepare for reading in reverse
test: flat_mutation_reader_test: test_reverse_reader_is_mutation_source: prepare for reading in reverse

The mutation source test suite will soon test reads in reverse. Prepare
for this by checking the reversed flag on the slice and not reversing
the data when set. The test will have two modes effectively:
* Forward mode: data is reversed before read, the reversed again during
  read.
* Reverse mode: data is already reversed and it is reversed back during
  read.

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