tests: add parsing varint from JSON string test
tests: add parsing varint from JSON string test

Refs #3666
Message-Id: <01ed2f00fab5f3a26f1e8c5615fe5e9fcdd3812e@scylladb.com>

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