test/alternator: test TTL expiration for table with sort key
test/alternator: test TTL expiration for table with sort key

The basic test for TTL expiration, test_ttl.py::test_ttl_expiration,
uses a table with only a partition key. Most of the item expiration
logic is exactly the same for tables that also have a sort key, but
the step of *deleting* the item is different, so let's add a test
that verifies that also in this case, the expired item is properly
deleted.

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

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