test/alternator: additional test for BatchGetItem
test/alternator: additional test for BatchGetItem

Our simple test for BatchGetItem on a table with sort keys still has
requests with just one sort key per partition, so if BatchGetItem has
a bug with requesting multiple sort keys from the same partition,
such bug won't be caught by the simple tests. So in this test we add a
test that does. This will be useful for the next patch, we are planning
to refactor BatchGetItem's handling of multiple sort keys in the same
partition - so it will be useful to have more regression tests.

The tests test_batch_get_item_large and test_batch_get_item_partial
would actually also catch such bugs, but they are more elaborate tests
and it's nice to have smaller tests more focused on checking specific
features.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

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