gettext: Missing tests for fallback translations

[tomasr8]

For NullTranslations and GNUTranslations instances, it is possible to add a fallback that is invoked when a translation is not found via t.add_fallback(fallback). However, this is currently not tested, neither for NullTranslations, nor for GNUTranslations. We should test this behaviour.

Linked PRs

• gh-130320: gettext: Test fallback translations #130321
• [3.13] gh-130320: gettext: Test fallback translations (GH-130321) #132359