Trying to run the test with more than one shard results in a failure
when generating sharding metadata:
```
ERROR 2025-08-27 16:00:17,551 [shard 0:main] table - Memtable flush failed due to: std::runtime_error (Failed to generate sharding metadata for /tmp/scylla-c9fa42fe/ks/cf-2938a030834e11f0a561ffa33feb022d/me-3gt6_12wh_1gifk2ijgeu1ovc1m5-big-Data.db). Aborting
```
Let's require that the test be run with a single shard.
Closesscylladb/scylladb#25703