Currently, all the mutations this test generates are applied on shard 0.
In rare cases, this may lead to the following crash, when the flushed
sstable doesn't contain any key that belongs to the current shard,
as seen in https://jenkins.scylladb.com/job/releng/job/Scylla-CI/1390/artifact/testlog/x86_64/dev/database_test.test_truncate_without_snapshot_during_writes.114.log
```
WARN 2022-07-17 17:41:36,630 [shard 0] sstable - create_sharding_metadata: range=[{-468459073612751032, pk{00046b657930}}, {-468459073612751032, pk{00046b657930}}] has no intersection with shard=0 first_key={key: pk{00046b657930}, token:-468459073612751032} last_key={key: pk{00046b657930}, token:-468459073612751032} ranges_single_shard=[] ranges_all_shards={{1, {[{-468459073612751032, pk{00046b657930}}, {-468459073612751032, pk{00046b657930}}]}}}
ERROR 2022-07-17 17:41:36,630 [shard 0] table - failed to write sstable /jenkins/workspace/releng/Scylla-CI/scylla/testlog/x86_64/dev/scylla-e2b694c7-db4f-4f9d-9940-9c6c21850888/ks/cf-8f74aba005de11ed92fa8661a0ed7890/me-2-big-Data.db: std::runtime_error (Failed to generate sharding metadata for /jenkins/workspace/releng/Scylla-CI/scylla/testlog/x86_64/dev/scylla-e2b694c7-db4f-4f9d-9940-9c6c21850888/ks/cf-8f74aba005de11ed92fa8661a0ed7890/me-2-big-Data.db)
ERROR 2022-07-17 17:41:36,631 [shard 0] table - Memtable flush failed due to: std::runtime_error (Failed to generate sharding metadata for /jenkins/workspace/releng/Scylla-CI/scylla/testlog/x86_64/dev/scylla-e2b694c7-db4f-4f9d-9940-9c6c21850888/ks/cf-8f74aba005de11ed92fa8661a0ed7890/me-2-big-Data.db). Aborting, at 0x329e28e 0x329e780 0x329ea88 0xf5bc69 0xf956b1 0x3196dc4 0x3198037 0x319742a 0x32be2e4 0x32bd8e1 0x32ba01c 0x317f97d /lib64/libpthread.so.0+0x92a4 /lib64/libc.so.6+0x100322
```
Instead, generate random keys and apply them on their
owning shard, and truncate all database shards.
Fixes#11076
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
1c26d49fba