alternator-test: add test case for GSI with both keys
alternator-test: add test case for GSI with both keys

A case which adds a global secondary index on a table with both
hash and sort keys is added.

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