Increase test coverage of `gettext.c2py`

[tomasr8]

gettext.c2py is not fully covered:

(_parse is called by c2py)
We need to cover negation (e.g. !n) and an invalid ternary (e.g. n ? 1 2)

Linked PRs

• gh-130193: Increase test coverage of gettext.c2py #130208
• [3.13] gh-130193: Increase test coverage of gettext.c2py (GH-130208) #130217
• [3.12] gh-130193: Increase test coverage of gettext.c2py (GH-130208) #130218