test: add a case for disengaged optional values in system tables
test: add a case for disengaged optional values in system tables

Following the patch which fixes incorrect access to disengaged
optionals, a test case which used to reproduce the problem is added.
Message-Id: <7c8a486b52d1a0525528752e41b2389950b3b629@scylladb.com>

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