Increase coverage of `json` module

[hugovk]

I'll open a PR to increase some test coverage, following https://devguide.python.org/testing/coverage/

For my own reference, here's a one-liner to run tests and generate HTML coverage report:

python -m coverage run --pylib --branch --source=json Lib/test/regrtest.py test_json -v && python -m coverage html -i --include="Lib/*" --omit="Lib/test/*,Lib/*/tests/*"

Linked PRs

• gh-115692: Add tests to increase json coverage #115693
• [3.12] gh-115692: Add tests to increase json coverage (GH-115693) #117867