Assorted minor cleanups in the test_json_parser module

Andrew Dunstan <andrew@dunslane.net>

Commit: 42fa4b660143b66bea1fb90793ec90054e170c93
Author: Andrew Dunstan <andrew@dunslane.net>
Date: 2024-04-12T14:32:30Z
Releases: 17.0
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.

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