tests/row_cache_alloc_stress: use large cells instead of many rows
tests/row_cache_alloc_stress: use large cells instead of many rows

With streamed_mutations a partition with many small rows doesn't stress
the cache as much as the test expects. Use large clustering rows instead.

Signed-off-by: Paweł Dziepak <e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>

avikivity/scylla,kjniemi/scylla,duarten/scylla,kjniemi/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,raphaelsc/scylla,duarten/scylla,raphaelsc/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,raphaelsc/scylla,avikivity/scylla