tests/flat_mutation_reader_assertions: fix fast forwarding
tests/flat_mutation_reader_assertions: fix fast forwarding

Both fast_forward_to() overloads return a future which should be waited
for. Additionally, fast_forward_to(const dht::partition_range&) expects
the range to remain valid at least until the next call to
fast_forward_to(). The original mutation_reader_assertions guaranteed
that and so should flat_mutation_reader_assertions.

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