tests: test_multishard_combining_reader_as_mutation_source: don't copy mutations cross shard
tests: test_multishard_combining_reader_as_mutation_source: don't copy mutations cross shard

It's illegal. Freeze-unfreeze them instead when crossing shard
boundaries.

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