tests/mutation_source: add test for complex fast_forwarding and slicing
tests/mutation_source: add test for complex fast_forwarding and slicing

While we already had tests that verified inter- and intra-partition
fast-forwarding as well as slicing, they had quite limited scope and
didn't combine those operations. The new test is meant to extensively
test these cases.

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