tests/mutation_reader_test: add test_multishard_combining_reader_next_partition
tests/mutation_reader_test: add test_multishard_combining_reader_next_partition

Test the interaction of the multishard reader with the foreign reader
w.r.t next_partition(). next_partition() is a special operation, as it
its execution is deferred until the next cross-shard operations. Give it
some extra stress-testing.

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