Assorted minor cleanups in the test_json_parser module
Per gripes from Michael Paquier
Discussion: https://postgr.es/m/ZhTQ6_w1vwOhqTQI@paquier.xyz
Along the way, also clean up a handful of typos in
3311ea86ed and
ea7b4e9a2a, found by Alexander Lakhin, and a couple of stylistic
snafus noted by Daniel Westermann and Daniel Gustafsson.