json_formatter_test: Add rudimentary json formatter test
json_formatter_test: Add rudimentary json formatter test

This one got lost at sea in the last patch. How embarrassing.

Message-Id: <759ab59a3ab63d7322b6484a71595b622c98ff04@scylladb.com>

avikivity/seastar,syuu1228/seastar,syuu1228/seastar,cloudius-systems/seastar,dreamsxin/seastar,cloudius-systems/seastar,scylladb/seastar,cloudius-systems/seastar,scylladb/seastar,avikivity/seastar,syuu1228/seastar,scylladb/seastar,dreamsxin/seastar,avikivity/seastar,dreamsxin/seastar