tests: add secondary index paging unit test case
tests: add secondary index paging unit test case

A simple case for SI paging is added to secondary_index_test suite.
This commit should be followed by more complex testing
and serves as an example on how to extract paging state and use it
across CQL queries.
Message-Id: <637c3f5e5fb70d51d7c4e1abbb53b055e5ee729c@scylladb.com>

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