tests: reduce multishard_mutation_test runtime in debug mode
tests: reduce multishard_mutation_test runtime in debug mode

Debug mode is so slow that generating 1000 mutations is too much for it.
High memory use can also confuse the santitizers that track each allocation.

Reduce mutation count from 1000 to 10 in debug mode.

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